This commit adds grace-period sequence numbers (->gp_seq) to the
rcu_state, rcu_node, and rcu_data structures, and updates them.
It also checks for consistency between rsp->gpnum and rsp->gp_seq.
These ->gp_seq counters will eventually replace the existing ->gpnum
and ->completed counters, allowing a single memory access to determine
whether or not a grace period is in progress and if so, which one.
This in turn will enable changes that will reduce ->lock contention on
the leaf rcu_node structures.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
At the end of rcu_gp_cleanup(), if another grace period is needed, but
not via rcu_accelerate_cbs(), the ->gp_flags field is written twice,
once when making the new grace-period request, and once when clearing
all other types of requests. This commit therefore adds an else-clause
to avoid this double write.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit causes a splat if RCU is idle and a request for a new grace
period is ignored for more than one second. This splat normally indicates
that some code path asked for a new grace period, but failed to wake up
the RCU grace-period kthread.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
[ paulmck: Fix bug located by Dan Carpenter and his static checker. ]
[ paulmck: Fix self-deadlock bug located 0day test robot. ]
[ paulmck: Disable unless CONFIG_PROVE_RCU=y. ]
There is a two-jiffy delay between the time that a CPU will self-report
an RCU CPU stall warning and the time that some other CPU will report a
warning on behalf of the first CPU. This has worked well in the past,
but on busy systems, it is possible for the two warnings to overlap,
which makes interpreting them extremely difficult.
This commit therefore uses a cmpxchg-based timing decision that
allows only one report in a given one-minute period (assuming default
stall-warning Kconfig parameters). This approach will of course fail
if you are seeing minute-long vCPU preemption, but in that case the
overlapping RCU CPU stall warnings are the least of your worries.
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
The WARN_ON_ONCE(rcu_preempt_blocked_readers_cgp()) in
rcu_gp_cleanup() triggers (inexplicably, of course) every so often.
This commit therefore extracts more information.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
The x86/mtrr code does horrific things because hardware. It uses
stop_machine_from_inactive_cpu(), which does a wakeup (of the stopper
thread on another CPU), which uses RCU, all before the CPU is onlined.
RCU complains about this, because wakeups use RCU and RCU does
(rightfully) not consider offline CPUs for grace-periods.
Fix this by initializing RCU way early in the MTRR case.
Tested-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
[ paulmck: Add !SMP support, per 0day Test Robot report. ]
Now that grace-period requests use funnel locking and now that they
set ->gp_flags to RCU_GP_FLAG_INIT even when the RCU grace-period
kthread has not yet started, rcu_gp_kthread() no longer needs to check
need_any_future_gp() at startup time. This commit therefore removes
this check.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>
Now that RCU no longer relies on failsafe checks, cpu_needs_another_gp()
can be greatly simplified. This simplification eliminates the last
call to rcu_future_needs_gp() and to rcu_segcblist_future_gp_needed(),
both of which which can then be eliminated. And then, because
cpu_needs_another_gp() is called only from __rcu_pending(), it can be
inlined and eliminated.
This commit carries out the simplification, inlining, and elimination
called out above.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>
All of the cpu_needs_another_gp() function's checks (except for
newly arrived callbacks) have been subsumed into the rcu_gp_cleanup()
function's scan of the rcu_node tree. This commit therefore drops the
call to cpu_needs_another_gp(). The check for newly arrived callbacks
is supplied by rcu_accelerate_cbs(). Any needed advancing (as in the
earlier rcu_advance_cbs() call) will be supplied when the corresponding
CPU becomes aware of the end of the now-completed grace period.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>
If rcu_start_this_gp() is invoked with a requested grace period more
than three in the future, then either the ->need_future_gp[] array
needs to be bigger or the caller needs to be repaired. This commit
therefore adds a WARN_ON_ONCE() checking for this condition.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>
The rcu_start_this_gp() function had a simple form of funnel locking that
used only the leaves and root of the rcu_node tree, which is fine for
systems with only a few hundred CPUs, but sub-optimal for systems having
thousands of CPUs. This commit therefore adds full-tree funnel locking.
This variant of funnel locking is unusual in the following ways:
1. The leaf-level rcu_node structure's ->lock is held throughout.
Other funnel-locking implementations drop the leaf-level lock
before progressing to the next level of the tree.
2. Funnel locking can be started at the root, which is convenient
for code that already holds the root rcu_node structure's ->lock.
Other funnel-locking implementations start at the leaves.
3. If an rcu_node structure other than the initial one believes
that a grace period is in progress, it is not necessary to
go further up the tree. This is because grace-period cleanup
scans the full tree, so that marking the need for a subsequent
grace period anywhere in the tree suffices -- but only if
a grace period is currently in progress.
4. It is possible that the RCU grace-period kthread has not yet
started, and this case must be handled appropriately.
However, the general approach of using a tree to control lock contention
is still in place.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>
The rcu_accelerate_cbs() function selects a grace-period target, which
it uses to have rcu_segcblist_accelerate() assign numbers to recently
queued callbacks. Then it invokes rcu_start_future_gp(), which selects
a grace-period target again, which is a bit pointless. This commit
therefore changes rcu_start_future_gp() to take the grace-period target as
a parameter, thus avoiding double selection. This commit also changes
the name of rcu_start_future_gp() to rcu_start_this_gp() to reflect
this change in functionality, and also makes a similar change to the
name of trace_rcu_future_gp().
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>
The rcu_start_gp_advanced() is invoked only from rcu_start_future_gp() and
much of its code is redundant when invoked from that context. This commit
therefore inlines rcu_start_gp_advanced() into rcu_start_future_gp(),
then removes rcu_start_gp_advanced().
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>
Once the grace period has ended, any RCU_GP_FLAG_FQS requests are
irrelevant: The grace period has ended, so there is no longer any
point in forcing quiescent states in order to try to make it end sooner.
This commit therefore causes rcu_gp_cleanup() to clear any bits other
than RCU_GP_FLAG_INIT from ->gp_flags at the end of the grace period.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>
It is true that currently only the low-order two bits are used, so
there should be no problem given modern machines and compilers, but
good hygiene and maintainability dictates use of an unsigned long
instead of an int. This commit therefore makes this change.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>
The __rcu_process_callbacks() function currently checks to see if
the current CPU needs a grace period and also if there is any other
reason to kick off a new grace period. This is one of the fail-safe
checks that has been rendered unnecessary by the changes that increase
the accuracy of rcu_gp_cleanup()'s estimate as to whether another grace
period is required. Because this particular fail-safe involved acquiring
the root rcu_node structure's ->lock, which has seen excessive contention
in real life, this fail-safe needs to go.
However, one check must remain, namely the check for newly arrived
RCU callbacks that have not yet been associated with a grace period.
One might hope that the checks in __note_gp_changes(), which is invoked
indirectly from rcu_check_quiescent_state(), would suffice, but this
function won't be invoked at all if RCU is idle. It is therefore necessary
to replace the fail-safe checks with a simpler check for newly arrived
callbacks during an RCU idle period, which is exactly what this commit
does. This change removes the final call to rcu_start_gp(), so this
function is removed as well.
Note that lockless use of cpu_needs_another_gp() is racy, but that
these races are harmless in this case. If RCU really is idle, the
values will not change, so the return value from cpu_needs_another_gp()
will be correct. If RCU is not idle, the resulting redundant call to
rcu_accelerate_cbs() will be harmless, and might even have the benefit
of reducing grace-period latency a bit.
This commit also moves interrupt disabling into the "if" statement to
improve real-time response a bit.
Reported-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>
When __call_rcu_core() notices excessive numbers of callbacks pending
on the current CPU, we know that at least one of them is not yet
classified, namely the one that was just now queued. Therefore, it
is not necessary to invoke rcu_start_gp() and thus not necessary to
acquire the root rcu_node structure's ->lock. This commit therefore
replaces the rcu_start_gp() with rcu_accelerate_cbs(), thus replacing
an acquisition of the root rcu_node structure's ->lock with that of
this CPU's leaf rcu_node structure.
This decreases contention on the root rcu_node structure's ->lock.
Reported-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>
The rcu_migrate_callbacks() function invokes rcu_advance_cbs()
twice, ignoring the return value. This is OK at pressent because of
failsafe code that does the wakeup when needed. However, this failsafe
code acquires the root rcu_node structure's lock frequently, while
rcu_migrate_callbacks() does so only once per CPU-offline operation.
This commit therefore makes rcu_migrate_callbacks()
wake up the RCU GP kthread when either call to rcu_advance_cbs()
returns true, thus removing need for the failsafe code.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>
There is no longer any need for ->need_future_gp[] to count the number of
requests for future grace periods, so this commit converts the additions
to assignments to "true" and reduces the size of each element to one byte.
While we are in the area, fix an obsolete comment.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>
Currently, the rcu_future_needs_gp() function checks only the current
element of the ->need_future_gps[] array, which might miss elements that
were offset from the expected element, for example, due to races with
the start or the end of a grace period. This commit therefore makes
rcu_future_needs_gp() use the need_any_future_gp() macro to check all
of the elements of this array.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>
Currently, rcu_gp_cleanup() scans the rcu_node tree in order to reset
state to reflect the end of the grace period. It also checks to see
whether a new grace period is needed, but in a number of cases, rather
than directly cause the new grace period to be immediately started, it
instead leaves the grace-period-needed state where various fail-safes
can find it. This works fine, but results in higher contention on the
root rcu_node structure's ->lock, which is undesirable, and contention
on that lock has recently become noticeable.
This commit therefore makes rcu_gp_cleanup() immediately start a new
grace period if there is any need for one.
It is quite possible that it will later be necessary to throttle the
grace-period rate, but that can be dealt with when and if.
Reported-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>
The rcu_gp_kthread() function immediately sleeps waiting to be notified
of the need for a new grace period, which currently works because there
are a number of code sequences that will provide the needed wakeup later.
However, some of these code sequences need to acquire the root rcu_node
structure's ->lock, and contention on that lock has started manifesting.
This commit therefore makes rcu_gp_kthread() check for early-boot activity
when it starts up, omitting the initial sleep in that case.
Reported-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>
Accessors for the ->need_future_gp[] array are currently open-coded,
which makes them difficult to change. To improve maintainability, this
commit adds need_future_gp_mask() to compute the indexing mask from the
array size, need_future_gp_element() to access the element corresponding
to the specified grace-period number, and need_any_future_gp() to
determine if any future grace period is needed. This commit also applies
need_future_gp_element() to existing open-coded single-element accesses.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>
The rcu_start_future_gp() function uses a sloppy check for a grace
period being in progress, which works today because there are a number
of code sequences that resolve the resulting races. However, some of
these race-resolution code sequences must acquire the root rcu_node
structure's ->lock, and contention on that lock has started manifesting.
This commit therefore makes rcu_start_future_gp() check more precise,
eliminating the sloppy lockless check of the rcu_state structure's ->gpnum
and ->completed fields. The effect is that rcu_start_future_gp() will
sometimes unnecessarily attempt to start a new grace period, but this
overhead will be reduced later using funnel locking.
Reported-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>
When rcu_cbs_completed() is invoked on a non-root rcu_node structure,
it unconditionally assumes that two grace periods must complete before
the callbacks at hand can be invoked. This is overly conservative because
if that non-root rcu_node structure believes that no grace period is in
progress, and if the corresponding rcu_state structure's ->gpnum field
has not yet been incremented, then these callbacks may safely be invoked
after only one grace period has completed.
This change is required to permit grace-period start requests to use
funnel locking, which is in turn permitted to reduce root rcu_node ->lock
contention, which has been observed by Nick Piggin. Furthermore, such
contention will likely be increased by the merging of RCU-bh, RCU-preempt,
and RCU-sched, so it makes sense to take steps to decrease it.
This commit therefore improves the accuracy of rcu_cbs_completed() when
invoked on a non-root rcu_node structure as described above.
Reported-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>
This commit adds rcu_first_leaf_node() that returns a pointer to
the first leaf rcu_node structure in the specified RCU flavor and an
rcu_is_leaf_node() that returns true iff the specified rcu_node structure
is a leaf. This commit also uses these macros where appropriate.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>
Commit e31d28b6ab ("trace: Eliminate cond_resched_rcu_qs() in favor
of cond_resched()") substituted cond_resched() for the earlier call
to cond_resched_rcu_qs(). However, the new-age cond_resched() does
not do anything to help RCU-tasks grace periods because (1) RCU-tasks
is only enabled when CONFIG_PREEMPT=y and (2) cond_resched() is a
complete no-op when preemption is enabled. This situation results
in hangs when running the trace benchmarks.
A number of potential fixes were discussed on LKML
(https://lkml.kernel.org/r/20180224151240.0d63a059@vmware.local.home),
including making cond_resched() not be a no-op; making cond_resched()
not be a no-op, but only when running tracing benchmarks; reverting
the aforementioned commit (which works because cond_resched_rcu_qs()
does provide an RCU-tasks quiescent state; and adding a call to the
scheduler/RCU rcu_note_voluntary_context_switch() function. All were
deemed unsatisfactory, either due to added cond_resched() overhead or
due to magic functions inviting cargo culting.
This commit renames cond_resched_rcu_qs() to cond_resched_tasks_rcu_qs(),
which provides a clear hint as to what this function is doing and
why and where it should be used, and then replaces the call to
cond_resched() with cond_resched_tasks_rcu_qs() in the trace benchmark's
benchmark_event_kthread() function.
Reported-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>
The latency of RCU expedited grace periods grows with increasing numbers
of CPUs, eventually failing to be all that expedited. Much of the growth
in latency is in the initialization phase, so this commit uses workqueues
to carry out this initialization concurrently on a rcu_node-by-rcu_node
basis.
This change makes use of a new rcu_par_gp_wq because flushing a work
item from another work item running from the same workqueue can result
in deadlock.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Nicholas Piggin <npiggin@gmail.com>
RCU's expedited grace periods can participate in out-of-memory deadlocks
due to all available system_wq kthreads being blocked and there not being
memory available to create more. This commit prevents such deadlocks
by allocating an RCU-specific workqueue_struct at early boot time, and
providing it with a rescuer to ensure forward progress. This uses the
shiny new init_rescuer() function provided by Tejun (but indirectly).
This commit also causes SRCU to use this new RCU-specific
workqueue_struct. Note that SRCU's use of workqueues never blocks them
waiting for readers, so this should be safe from a forward-progress
viewpoint. Note that this moves SRCU from system_power_efficient_wq
to a normal workqueue. In the unlikely event that this results in
measurable degradation, a separate power-efficient workqueue will be
creates for SRCU.
Reported-by: Prateek Sood <prsood@codeaurora.org>
Reported-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: Tejun Heo <tj@kernel.org>
Commits c0b334c5bf and ea9b0c8a26 introduced new sparse warnings
by accessing rcu_node->lock directly and ignoring the __private
marker. Introduce a new wrapper and use it. Also fix a similar problem
in srcutree.c introduced by a3883df393.
Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
It is not always obvious that the stack dump from a starved grace-period
kthread isn't instead that of a CPU stalling the current grace period.
This commit therefore adds a pr_err() flagging these dumps.
Reported-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
The debugfs interface displayed statistics on RCU-pending checks but
this interface has since been removed. This commit therefore removes the
no-longer-used rcu_state structure's ->n_force_qs_lh and ->n_force_qs_ngp
fields along with their updates. (Though the ->n_force_qs_ngp field
was actually not used at all, embarrassingly enough.)
If this information proves necessary in the future, the corresponding
event traces will be added.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
The debugfs interface displayed statistics on RCU-pending checks
but this interface has since been removed. This commit therefore
removes the no-longer-used rcu_data structure's ->n_rcu_pending,
->n_rp_core_needs_qs, ->n_rp_report_qs, ->n_rp_cb_ready,
->n_rp_cpu_needs_gp, ->n_rp_gp_completed, ->n_rp_gp_started,
->n_rp_nocb_defer_wakeup, and ->n_rp_need_nothing fields along with
their updates.
If this information proves necessary in the future, the corresponding
event traces will be added.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
The debugfs interface displayed statistics on RCU callback invocation but
this interface has since been removed. This commit therefore removes the
no-longer-used rcu_data structure's ->n_cbs_invoked and ->n_nocbs_invoked
fields along with their updates.
If this information proves necessary in the future, the corresponding
event traces will be added.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Since rcu_boot_init_percpu_data() is only called at boot time,
there is no data race and spinlock is not needed.
Signed-off-by: Lihao Liang <lianglihao@huawei.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
The following statement has for some reason proven non-intuitive:
WARN_ON_ONCE(rcu_segcblist_empty(&rdp->cblist) != (count == 0));
This commit therefore adds a comment that states that this warning
usually triggers in response to a double call_rcu(), which is sort
of like a double free. The comment also suggests building with
CONFIG_DEBUG_OBJECTS_RCU_HEAD=y to track down the double call_rcu().
Reported-by: David Howells <dhowells@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
The code that checks for non-idle non-nohz_idle-usermode tasks invoking
rcu_eqs_enter() and rcu_eqs_exit() prints a considerable quantity of
helpful information. However, these checks fire rarely, so the extra
complexity is no longer worth it. This commit therefore replaces this
debug code with simple WARN_ON_ONCE() statements.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
There is now only one call to rcu_eqs_exit_common() and there is no other
reason to keep it separate. This commit therefore inlines it into its
sole call site, saving a few lines of code in the process.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
There is now only one call to rcu_eqs_enter_common() and there is no other
reason to keep it separate. This commit therefore inlines it into its
sole call site, saving a few lines of code in the process.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Although ->dynticks_nesting is updated only by process level, it is
accessed from hardirq to check for interrupt-from-idle quiescent states.
Store tearing is thus possible, so this commit applies WRITE_ONCE()
to ->dynticks_nesting stores.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
The three RCU_LOCKDEP_WARN() calls in rcu_eqs_enter_common() are
redundant with other lockdep checks, so this commit removes them.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Because the ->dynticks_nesting field now only contains the process-based
nesting level instead of a value encoding both the process nesting level
and the irq "nesting" level, we no longer need a long long, even on
32-bit systems. This commit therefore changes both the ->dynticks_nesting
and ->dynticks_nmi_nesting fields to long.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Now that the irq path uses the rcu_nmi_{enter,exit}() algorithm,
rcu_irq_enter() and rcu_irq_exit() may be used from any context. There is
thus no need for rcu_irq_enter_disabled() and for the checks using it.
This commit therefore eliminates rcu_irq_enter_disabled().
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Now that ->dynticks_nesting counts only process-level dyntick-idle
entry and exit, there is no need for the elaborate segmented counter
with its guard fields and overflow checking. This commit therefore
makes ->dynticks_nesting be a simple counter.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
RCU currently uses two different mechanisms for tracking irqs and NMIs.
This is unnecessary complexity: Given that NMIs can nest and given that
RCU's tracking handles such nesting, the NMI tracking mechanism can also
be used to track irqs. This commit therefore defines rcu_irq_enter()
in terms of rcu_nmi_enter() and rcu_irq_exit() in terms of rcu_nmi_exit().
Unfortunately, callers must still distinguish between the irq and NMI
functions because additional actions are taken when an irq interrupts
idle or nohz_full usermode execution, and these actions cannot always
be taken from NMI handlers.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
In preparation for merging dyntick-idle irq handling into the NMI
algorithm, clamp ->dynticks_nmi_nesting value to allow for interrupts
that enter but never leave and vice versa.
It is important that the clamping happen outside of the extended quiescent
state. Otherwise, there will be short windows where irqs and NMIs fail
to convince RCU to start watching.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This is a code-motion-only commit that prepares to define rcu_irq_enter()
in terms of rcu_nmi_enter() and rcu_irq_exit() in terms of rcu_irq_exit().
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Both extended-quiescent-state entry and exit first update the nesting
counter and then adjust the dyntick-idle state. This means that there
are four states: (1) Both nesting and dyntick idle indicate idle,
(2) Nesting indicates idle but dyntick idle does not, (3) Nesting indicates
non-idle and dyntick idle does not, and (4) Both nesting and dyntick
idle indicate non-idle. This commit simplifies the state space by
eliminating #3, reversing the order of updates on exit from extended
quiescent state.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
NMIs can nest, and store tearing could in theory happen on carries
from one byte to the next. This commit therefore adds the WRITE_ONCE()
macros preventing this.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Pull core locking updates from Ingo Molnar:
"The main changes in this cycle are:
- Another attempt at enabling cross-release lockdep dependency
tracking (automatically part of CONFIG_PROVE_LOCKING=y), this time
with better performance and fewer false positives. (Byungchul Park)
- Introduce lockdep_assert_irqs_enabled()/disabled() and convert
open-coded equivalents to lockdep variants. (Frederic Weisbecker)
- Add down_read_killable() and use it in the VFS's iterate_dir()
method. (Kirill Tkhai)
- Convert remaining uses of ACCESS_ONCE() to
READ_ONCE()/WRITE_ONCE(). Most of the conversion was Coccinelle
driven. (Mark Rutland, Paul E. McKenney)
- Get rid of lockless_dereference(), by strengthening Alpha atomics,
strengthening READ_ONCE() with smp_read_barrier_depends() and thus
being able to convert users of lockless_dereference() to
READ_ONCE(). (Will Deacon)
- Various micro-optimizations:
- better PV qspinlocks (Waiman Long),
- better x86 barriers (Michael S. Tsirkin)
- better x86 refcounts (Kees Cook)
- ... plus other fixes and enhancements. (Borislav Petkov, Juergen
Gross, Miguel Bernal Marin)"
* 'locking-core-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: (70 commits)
locking/x86: Use LOCK ADD for smp_mb() instead of MFENCE
rcu: Use lockdep to assert IRQs are disabled/enabled
netpoll: Use lockdep to assert IRQs are disabled/enabled
timers/posix-cpu-timers: Use lockdep to assert IRQs are disabled/enabled
sched/clock, sched/cputime: Use lockdep to assert IRQs are disabled/enabled
irq_work: Use lockdep to assert IRQs are disabled/enabled
irq/timings: Use lockdep to assert IRQs are disabled/enabled
perf/core: Use lockdep to assert IRQs are disabled/enabled
x86: Use lockdep to assert IRQs are disabled/enabled
smp/core: Use lockdep to assert IRQs are disabled/enabled
timers/hrtimer: Use lockdep to assert IRQs are disabled/enabled
timers/nohz: Use lockdep to assert IRQs are disabled/enabled
workqueue: Use lockdep to assert IRQs are disabled/enabled
irq/softirqs: Use lockdep to assert IRQs are disabled/enabled
locking/lockdep: Add IRQs disabled/enabled assertion APIs: lockdep_assert_irqs_enabled()/disabled()
locking/pvqspinlock: Implement hybrid PV queued/unfair locks
locking/rwlocks: Fix comments
x86/paravirt: Set up the virt_spin_lock_key after static keys get initialized
block, locking/lockdep: Assign a lock_class per gendisk used for wait_for_completion()
workqueue: Remove now redundant lock acquisitions wrt. workqueue flushes
...
Lockdep now has an integrated IRQs disabled/enabled sanity check. Just
use it instead of the ad-hoc RCU version.
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: David S . Miller <davem@davemloft.net>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Tejun Heo <tj@kernel.org>
Link: http://lkml.kernel.org/r/1509980490-4285-15-git-send-email-frederic@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Because many of RCU's files have not been included into docbook, a
number of errors have accumulated. This commit fixes them.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
If you add or remove calls to rcu_idle_enter(), rcu_user_enter(),
rcu_irq_exit(), rcu_irq_exit_irqson(), rcu_idle_exit(), rcu_user_exit(),
rcu_irq_enter(), rcu_irq_enter_irqson(), rcu_nmi_enter(), or
rcu_nmi_exit(), you should run a full set of tests on a kernel built
with CONFIG_RCU_EQS_DEBUG=y.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
One common question upon seeing an RCU CPU stall warning is "did
the stalled CPUs have interrupts disabled?" However, the current
stall warnings are silent on this point. This commit therefore
uses irq_work to check whether stalled CPUs still respond to IPIs,
and flags this state in the RCU CPU stall warning console messages.
Reported-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
There is some confusion as to which of cond_resched() or
cond_resched_rcu_qs() should be added to long in-kernel loops.
This commit therefore eliminates the decision by adding RCU quiescent
states to cond_resched(). This commit also simplifies the code that
used to interact with cond_resched_rcu_qs(), and that now interacts with
cond_resched(), to reduce its overhead. This reduction is necessary to
allow the heavier-weight cond_resched_rcu_qs() mechanism to be invoked
everywhere that cond_resched() is invoked.
Part of that reduction in overhead converts the jiffies_till_sched_qs
kernel parameter to read-only at runtime, thus eliminating the need for
bounds checking.
Reported-by: Michal Hocko <mhocko@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
[ paulmck: Keep PREEMPT=n cond_resched a no-op, per Peter Zijlstra. ]
- A memory fix with left over code from spliting out ftrace_ops
and function graph tracer, where the function graph tracer could
reset the trampoline pointer, leaving the old trampoline not to
be freed (memory leak).
- The update to Paul's patch that added the unnecessary READ_ONCE().
This removes the unnecessary READ_ONCE() instead of having to rebase
the branch to update the patch that added it.
-----BEGIN PGP SIGNATURE-----
iQFIBAABCAAyFiEEQEw9Eu0DdyUUkuUUybkF8mrZjcsFAlnU++sUHHJvc3RlZHRA
Z29vZG1pcy5vcmcACgkQybkF8mrZjcujzgf/ebIzGKe5vQKNrL4ITAcIz0T7Hvzl
pWw4uJp8kqO9x9EHMnztAkltQigvjvgDKZozJpUGgtNsFLuvdgQSBMK24YV8vLHs
UmXEnQ2tSB/2Sg2ccEnpjVXaMzL9aqlbeTmACbdd9UgZnvPiUYPejq2jFfECFQjb
k/gZT911ukBtx4mXYKzGFbTEZHdc/YUs6Y/wzB1ox5BBIUh71ZDZXxQTUHfXHlwS
Cst69/9dKl4nBEGDGas6/95iR+ORVv85osI/pqPtjSj4EkRnWfVRotaH1kNuSQil
gDIHSoy35NfXJx77/5IFHfrjFBAkr0IYRNL/jZaWazwM7rdqfAN8TwMQuA==
=4CtF
-----END PGP SIGNATURE-----
Merge tag 'trace-v4.14-rc1-3' of git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-trace
Pull tracing fixlets from Steven Rostedt:
"Two updates:
- A memory fix with left over code from spliting out ftrace_ops and
function graph tracer, where the function graph tracer could reset
the trampoline pointer, leaving the old trampoline not to be freed
(memory leak).
- The update to Paul's patch that added the unnecessary READ_ONCE().
This removes the unnecessary READ_ONCE() instead of having to
rebase the branch to update the patch that added it"
* tag 'trace-v4.14-rc1-3' of git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-trace:
rcu: Remove extraneous READ_ONCE()s from rcu_irq_{enter,exit}()
ftrace: Fix kmemleak in unregister_ftrace_graph
The read of ->dynticks_nmi_nesting in rcu_irq_enter() and rcu_irq_exit()
is currently protected with READ_ONCE(). However, this protection is
unnecessary because (1) ->dynticks_nmi_nesting is updated only by the
current CPU, (2) Although NMI handlers can update this field, they reset
it back to its old value before return, and (3) Interrupts are disabled,
so nothing else can modify it. The value of ->dynticks_nmi_nesting is
thus effectively constant, and so no protection is required.
This commit therefore removes the READ_ONCE() protection from these
two accesses.
Link: http://lkml.kernel.org/r/20170926031902.GA2074@linux.vnet.ibm.com
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
has been pointing out constant problems. The changes have been going into
the stack tracer, but it has been discovered that the problem isn't
with the stack tracer itself, but it is with calling save_stack_trace()
from within the internals of RCU. The stack tracer is the one that
can trigger the issue the easiest, but examining the problem further,
it could also happen from a WARN() in the wrong place, or even if
an NMI happened in this area and it did an rcu_read_lock().
The critical area is where RCU is not watching. Which can happen while
going to and from idle, or bringing up or taking down a CPU.
The final fix was to put the protection in kernel_text_address() as it
is the one that requires RCU to be watching while doing the stack trace.
To make this work properly, Paul had to allow rcu_irq_enter() happen after
rcu_nmi_enter(). This should have been done anyway, since an NMI can
page fault (reading vmalloc area), and a page fault triggers rcu_irq_enter().
One patch is just a consolidation of code so that the fix only needed
to be done in one location.
-----BEGIN PGP SIGNATURE-----
iQFIBAABCAAyFiEEQEw9Eu0DdyUUkuUUybkF8mrZjcsFAlnGyXoUHHJvc3RlZHRA
Z29vZG1pcy5vcmcACgkQybkF8mrZjctKtwf8CeKGqOdlqkZEafIpWaIASXmAVMO/
WE+hQK+rCydWFvzADgb/rOmsR0ou8WGEXcuUPxVxmvMyqhKhZ6AU1hE/7Y8P0pMq
F4bev+j3lAJC65ezFAh+ZQcIjaRIH4MFVPsUTaibSPSN7xziMNIpbf9VOVfpUm8A
jf9p6YAmyhFVi6DstCc29SWnywEVwC2ZWRVKRPXKry8/dPxjfVcLclGX680Eqi9I
EnYaOdC/mGbtvHPOUSs/P0cfxExHmyEErQHeOV8FPymj6KJ6+KoYIiELNlTHUBj/
eeKzrHc/b3j+lz0RPlA8WxYmpmEm4SE5cV3vRebdBNUBrABSN1RxeOozyQ==
=1KkS
-----END PGP SIGNATURE-----
Merge tag 'trace-v4.14-rc1-2' of git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-trace
Pull tracing fixes from Steven Rostedt:
"Stack tracing and RCU has been having issues with each other and
lockdep has been pointing out constant problems.
The changes have been going into the stack tracer, but it has been
discovered that the problem isn't with the stack tracer itself, but it
is with calling save_stack_trace() from within the internals of RCU.
The stack tracer is the one that can trigger the issue the easiest,
but examining the problem further, it could also happen from a WARN()
in the wrong place, or even if an NMI happened in this area and it did
an rcu_read_lock().
The critical area is where RCU is not watching. Which can happen while
going to and from idle, or bringing up or taking down a CPU.
The final fix was to put the protection in kernel_text_address() as it
is the one that requires RCU to be watching while doing the stack
trace.
To make this work properly, Paul had to allow rcu_irq_enter() happen
after rcu_nmi_enter(). This should have been done anyway, since an NMI
can page fault (reading vmalloc area), and a page fault triggers
rcu_irq_enter().
One patch is just a consolidation of code so that the fix only needed
to be done in one location"
* tag 'trace-v4.14-rc1-2' of git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-trace:
tracing: Remove RCU work arounds from stack tracer
extable: Enable RCU if it is not watching in kernel_text_address()
extable: Consolidate *kernel_text_address() functions
rcu: Allow for page faults in NMI handlers
A number of architecture invoke rcu_irq_enter() on exception entry in
order to allow RCU read-side critical sections in the exception handler
when the exception is from an idle or nohz_full CPU. This works, at
least unless the exception happens in an NMI handler. In that case,
rcu_nmi_enter() would already have exited the extended quiescent state,
which would mean that rcu_irq_enter() would (incorrectly) cause RCU
to think that it is again in an extended quiescent state. This will
in turn result in lockdep splats in response to later RCU read-side
critical sections.
This commit therefore causes rcu_irq_enter() and rcu_irq_exit() to
take no action if there is an rcu_nmi_enter() in effect, thus avoiding
the unscheduled return to RCU quiescent state. This in turn should
make the kernel safe for on-demand RCU voyeurism.
Link: http://lkml.kernel.org/r/20170922211022.GA18084@linux.vnet.ibm.com
Cc: stable@vger.kernel.org
Fixes: 0be964be0 ("module: Sanitize RCU usage and locking")
Reported-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
First, number of CPUs can't be negative number.
Second, different signnnedness leads to suboptimal code in the following
cases:
1)
kmalloc(nr_cpu_ids * sizeof(X));
"int" has to be sign extended to size_t.
2)
while (loff_t *pos < nr_cpu_ids)
MOVSXD is 1 byte longed than the same MOV.
Other cases exist as well. Basically compiler is told that nr_cpu_ids
can't be negative which can't be deduced if it is "int".
Code savings on allyesconfig kernel: -3KB
add/remove: 0/0 grow/shrink: 25/264 up/down: 261/-3631 (-3370)
function old new delta
coretemp_cpu_online 450 512 +62
rcu_init_one 1234 1272 +38
pci_device_probe 374 399 +25
...
pgdat_reclaimable_pages 628 556 -72
select_fallback_rq 446 369 -77
task_numa_find_cpu 1923 1807 -116
Link: http://lkml.kernel.org/r/20170819114959.GA30580@avx2
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
The rcu_idle_exit() and rcu_idle_enter() functions are exported because
they were originally used by RCU_NONIDLE(), which was intended to
be usable from modules. However, RCU_NONIDLE() now instead uses
rcu_irq_enter_irqson() and rcu_irq_exit_irqson(), which are not
exported, and there have been no complaints.
This commit therefore removes the exports from rcu_idle_exit() and
rcu_idle_enter().
Reported-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
All current callers of rcu_idle_enter() have irqs disabled, and
rcu_idle_enter() relies on this, but doesn't check. This commit
therefore adds a RCU_LOCKDEP_WARN() to add some verification to the trust.
While we are there, pass "true" rather than "1" to rcu_eqs_enter().
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
All callers to rcu_idle_enter() have irqs disabled, so there is no
point in rcu_idle_enter disabling them again. This commit therefore
replaces the irq disabling with a RCU_LOCKDEP_WARN().
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit adds assertions verifying the consistency of the rcu_node
structure's ->blkd_tasks list and its ->gp_tasks, ->exp_tasks, and
->boost_tasks pointers. In particular, the ->blkd_tasks lists must be
empty except for leaf rcu_node structures.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Set disable_rcu_irq_enter on not only rcu_eqs_enter_common() but also
rcu_eqs_exit(), since rcu_eqs_exit() suffers from the same issue as was
fixed for rcu_eqs_enter_common() by commit 03ecd3f48e ("rcu/tracing:
Add rcu_disabled to denote when rcu_irq_enter() will not work").
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
The _rcu_barrier_trace() function is a wrapper for trace_rcu_barrier(),
which needs TPS() protection for strings passed through the second
argument. However, it has escaped prior TPS()-ification efforts because
it _rcu_barrier_trace() does not start with "trace_". This commit
therefore adds the needed TPS() protection
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
These RCU waits were set to use interruptible waits to avoid the kthreads
contributing to system load average, even though they are not interruptible
as they are spawned from a kthread. Use the new TASK_IDLE swaits which makes
our goal clear, and removes confusion about these paths possibly being
interruptible -- they are not.
When the system is idle the RCU grace-period kthread will spend all its time
blocked inside the swait_event_interruptible(). If the interruptible() was
not used, then this kthread would contribute to the load average. This means
that an idle system would have a load average of 2 (or 3 if PREEMPT=y),
rather than the load average of 0 that almost fifty years of UNIX has
conditioned sysadmins to expect.
The same argument applies to swait_event_interruptible_timeout() use. The
RCU grace-period kthread spends its time blocked inside this call while
waiting for grace periods to complete. In particular, if there was only one
busy CPU, but that CPU was frequently invoking call_rcu(), then the RCU
grace-period kthread would spend almost all its time blocked inside the
swait_event_interruptible_timeout(). This would mean that the load average
would be 2 rather than the expected 1 for the single busy CPU.
Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>
Tested-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Luis R. Rodriguez <mcgrof@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
After adopting callbacks from a newly offlined CPU, the adopting CPU
checks to make sure that its callback list's count is zero only if the
list has no callbacks and vice versa. Unfortunately, it does so after
enabling interrupts, which means that false positives are possible due to
interrupt handlers invoking call_rcu(). Although these false positives
are improbable, rcutorture did make it happen once.
This commit therefore moves this check to an irq-disabled region of code,
thus suppressing the false positive.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Given that the rcu_state structure's >orphan_pend and ->orphan_done
fields are used only during migration of callbacks from the recently
offlined CPU to a surviving CPU, if rcu_send_cbs_to_orphanage() and
rcu_adopt_orphan_cbs() are combined, these fields can become local
variables in the combined function. This commit therefore combines
rcu_send_cbs_to_orphanage() and rcu_adopt_orphan_cbs() into a new
rcu_segcblist_merge() function and removes the ->orphan_pend and
->orphan_done fields.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
When migrating callbacks from a newly offlined CPU, we are already
holding the root rcu_node structure's lock, so it costs almost nothing
to advance and accelerate the newly migrated callbacks. This patch
therefore makes this advancing and acceleration happen.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
The ->orphan_lock is acquired and released only within the
rcu_migrate_callbacks() function, which now acquires the root rcu_node
structure's ->lock. This commit therefore eliminates the ->orphan_lock
in favor of the root rcu_node structure's ->lock.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
It is possible that the outgoing CPU is unaware of recent grace periods,
and so it is also possible that some of its pending callbacks are actually
ready to be invoked. The current callback-migration code would needlessly
force these callbacks to pass through another grace period. This commit
therefore invokes rcu_advance_cbs() on the outgoing CPU's callbacks in
order to give them full credit for having passed through any recent
grace periods.
This also fixes an odd theoretical bug where there are no callbacks in
the system except for those on the outgoing CPU, none of those callbacks
have yet been associated with a grace-period number, there is never again
another callback registered, and the surviving CPU never again takes a
scheduling-clock interrupt, never goes idle, and never enters nohz_full
userspace execution. Yes, this is (just barely) possible. It requires
that the surviving CPU be a nohz_full CPU, that its scheduler-clock
interrupt be shut off, and that it loop forever in the kernel. You get
bonus points if you can make this one happen! ;-)
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
RCU's CPU-hotplug callback-migration code first moves the outgoing
CPU's callbacks to ->orphan_done and ->orphan_pend, and only then
moves them to the NOCB callback list. This commit avoids the
extra step (and simplifies the code) by moving the callbacks directly
from the outgoing CPU's callback list to the NOCB callback list.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
The current CPU-hotplug RCU-callback-migration code checks
for the source (newly offlined) CPU being a NOCBs CPU down in
rcu_send_cbs_to_orphanage(). This commit simplifies callback migration a
bit by moving this check up to rcu_migrate_callbacks(). This commit also
adds a check for the source CPU having no callbacks, which eases analysis
of the rcu_send_cbs_to_orphanage() and rcu_adopt_orphan_cbs() functions.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
The rcu_node structure's ->n_cbs_orphaned and ->n_cbs_adopted fields
are updated, but never read. This commit therefore removes them.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
The update of the ->expmaskinitnext and of ->ncpus are unsynchronized,
with the value of ->ncpus being incremented long before the corresponding
->expmaskinitnext mask is updated. If an RCU expedited grace period
sees ->ncpus change, it will update the ->expmaskinit masks from the new
->expmaskinitnext masks. But it is possible that ->ncpus has already
been updated, but the ->expmaskinitnext masks still have their old values.
For the current expedited grace period, no harm done. The CPU could not
have been online before the grace period started, so there is no need to
wait for its non-existent pre-existing readers.
But the next RCU expedited grace period is in a world of hurt. The value
of ->ncpus has already been updated, so this grace period will assume
that the ->expmaskinitnext masks have not changed. But they have, and
they won't be taken into account until the next never-been-online CPU
comes online. This means that RCU will be ignoring some CPUs that it
should be paying attention to.
The solution is to update ->ncpus and ->expmaskinitnext while holding
the ->lock for the rcu_node structure containing the ->expmaskinitnext
mask. Because smp_store_release() is now used to update ->ncpus and
smp_load_acquire() is now used to locklessly read it, if the expedited
grace period sees ->ncpus change, then the updating CPU has to
already be holding the corresponding ->lock. Therefore, when the
expedited grace period later acquires that ->lock, it is guaranteed
to see the new value of ->expmaskinitnext.
On the other hand, if the expedited grace period loads ->ncpus just
before an update, earlier full memory barriers guarantee that
the incoming CPU isn't far enough along to be running any RCU readers.
This commit therefore makes the required change.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
RCU callbacks must be migrated away from an outgoing CPU, and this is
done near the end of the CPU-hotplug operation, after the outgoing CPU is
long gone. Unfortunately, this means that other CPU-hotplug callbacks
can execute while the outgoing CPU's callbacks are still immobilized
on the long-gone CPU's callback lists. If any of these CPU-hotplug
callbacks must wait, either directly or indirectly, for the invocation
of any of the immobilized RCU callbacks, the system will hang.
This commit avoids such hangs by migrating the callbacks away from the
outgoing CPU immediately upon its departure, shortly after the return
from __cpu_die() in takedown_cpu(). Thus, RCU is able to advance these
callbacks and invoke them, which allows all the after-the-fact CPU-hotplug
callbacks to wait on these RCU callbacks without risk of a hang.
While in the neighborhood, this commit also moves rcu_send_cbs_to_orphanage()
and rcu_adopt_orphan_cbs() under a pre-existing #ifdef to avoid including
dead code on the one hand and to avoid define-without-use warnings on the
other hand.
Reported-by: Jeffrey Hugo <jhugo@codeaurora.org>
Link: http://lkml.kernel.org/r/db9c91f6-1b17-6136-84f0-03c3c2581ab4@codeaurora.org
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Anna-Maria Gleixner <anna-maria@linutronix.de>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Richard Weinberger <richard@nod.at>
This commit augments the grace-period-kthread starvation debugging
messages by adding the last CPU that ran the kthread.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
The NO_HZ_FULL_SYSIDLE full-system-idle capability was added in 2013
by commit 0edd1b1784 ("nohz_full: Add full-system-idle state machine"),
but has not been used. This commit therefore removes it.
If it turns out to be needed later, this commit can always be reverted.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Ingo Molnar <mingo@kernel.org>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Anything that can be done with the RCU_KTHREAD_PRIO Kconfig option can
also be done with the rcutree.kthread_prio kernel boot parameter.
This commit therefore removes this Kconfig option.
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Rik van Riel <riel@redhat.com>
The RCU_TORTURE_TEST_SLOW_PREINIT, RCU_TORTURE_TEST_SLOW_PREINIT_DELAY,
RCU_TORTURE_TEST_SLOW_PREINIT_DELAY, RCU_TORTURE_TEST_SLOW_INIT,
RCU_TORTURE_TEST_SLOW_INIT_DELAY, RCU_TORTURE_TEST_SLOW_CLEANUP,
and RCU_TORTURE_TEST_SLOW_CLEANUP_DELAY Kconfig options are only
useful for torture testing, and there are the rcutree.gp_cleanup_delay,
rcutree.gp_init_delay, and rcutree.gp_preinit_delay kernel boot parameters
that rcutorture can use instead. The effect of these parameters is to
artificially slow down grace period initialization and cleanup in order
to make some types of race conditions happen more often.
This commit therefore simplifies Tree RCU a bit by removing the Kconfig
options and adding the corresponding kernel parameters to rcutorture's
.boot files instead. However, this commit also leaves out the kernel
parameters for TREE02, TREE04, and TREE07 in order to have about the
same number of tests slowed as not slowed. TREE01, TREE03, TREE05,
and TREE06 are slowed, and the rest are not slowed.
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
The "__call_rcu(): Leaked duplicate callback" error message from
__call_rcu() has proven to be unhelpful. This commit therefore changes
it to "__call_rcu(): Double-freed CB" and adds the value of the pointer
passed in. The value of the pointer improves debuggability by allowing
correlation with tracing output, for example, the rcu:rcu_callback trace
event.
Reported-by: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
The __rcu_is_watching() function is currently not used, aside from
to implement the rcu_is_watching() function. This commit therefore
eliminates __rcu_is_watching(), which has the beneficial side-effect
of shrinking include/linux/rcupdate.h a bit.
Reported-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
The include/linux/rcupdate.h file is included by more than 200
files, so shrinking it should provide some build-time benefits.
This commit therefore moves several docbook comments from rcupdate.h to
kernel/rcu/update.c, kernel/rcu/tree.c, and kernel/rcu/tree_plugin.h, thus
reducing the number of times that the compiler has to scan these comments.
This likely provides only a small benefit, but every little bit helps.
This commit also fixes a malformed bulleted list noted by the 0day
Test Robot.
Reported-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Comments can be helpful, but assertions carry more force. This
commit therefore adds lockdep_assert_held() and RCU_LOCKDEP_WARN()
calls to enforce lock-held and interrupt-disabled preconditions.
Reported-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit updates rcu_bootup_announce_oddness() to check additional
Kconfig options and module/boot parameters.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit adds WARN_ON_ONCE() calls that trigger if either
rcu_sched_qs() or rcu_bh_qs() are invoked with preemption enabled.
In the immortal words of Peter Zijlstra: "these are much harder to ignore
than comments".
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
The synchronize_kernel() primitive was removed in favor of
synchronize_sched() more than a decade ago, and it seems likely that
rather few kernel hackers are familiar with it. Its continued presence
is therefore providing more confusion than enlightenment. This commit
therefore removes the reference from the synchronize_sched() header
comment, and adds the corresponding information to the synchronize_rcu(0
header comment.
Reported-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Although preemptible RCU allows its read-side critical sections to be
preempted, general blocking is forbidden. The reason for this is that
excessive preemption times can be handled by CONFIG_RCU_BOOST=y, but a
voluntarily blocked task doesn't care how high you boost its priority.
Because preemptible RCU is a global mechanism, one ill-behaved reader
hurts everyone. Hence the prohibition against general blocking in
RCU-preempt read-side critical sections. Preemption yes, blocking no.
This commit enforces this prohibition.
There is a special exception for the -rt patchset (which they kindly
volunteered to implement): It is OK to block (as opposed to merely being
preempted) within an RCU-preempt read-side critical section, but only if
the blocking is subject to priority inheritance. This exception permits
CONFIG_RCU_BOOST=y to get -rt RCU readers out of trouble.
Why doesn't this exception also apply to mainline's rt_mutex? Because
of the possibility that someone does general blocking while holding
an rt_mutex. Yes, the priority boosting will affect the rt_mutex,
but it won't help with the task doing general blocking while holding
that rt_mutex.
Reported-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Currently rcu_barrier() uses call_rcu() to enqueue new callbacks
on each CPU with a non-empty callback list. This works, but means
that rcu_barrier() forces grace periods that are not otherwise needed.
The key point is that rcu_barrier() never needs to wait for a grace
period, but instead only for all pre-existing callbacks to be invoked.
This means that rcu_barrier()'s new callbacks should be placed in
the callback-list segment containing the last pre-existing callback.
This commit makes this change using the new rcu_segcblist_entrain()
function.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Pull RCU updates from Ingo Molnar:
"The main changes are:
- Debloat RCU headers
- Parallelize SRCU callback handling (plus overlapping patches)
- Improve the performance of Tree SRCU on a CPU-hotplug stress test
- Documentation updates
- Miscellaneous fixes"
* 'core-rcu-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: (74 commits)
rcu: Open-code the rcu_cblist_n_lazy_cbs() function
rcu: Open-code the rcu_cblist_n_cbs() function
rcu: Open-code the rcu_cblist_empty() function
rcu: Separately compile large rcu_segcblist functions
srcu: Debloat the <linux/rcu_segcblist.h> header
srcu: Adjust default auto-expediting holdoff
srcu: Specify auto-expedite holdoff time
srcu: Expedite first synchronize_srcu() when idle
srcu: Expedited grace periods with reduced memory contention
srcu: Make rcutorture writer stalls print SRCU GP state
srcu: Exact tracking of srcu_data structures containing callbacks
srcu: Make SRCU be built by default
srcu: Fix Kconfig botch when SRCU not selected
rcu: Make non-preemptive schedule be Tasks RCU quiescent state
srcu: Expedite srcu_schedule_cbs_snp() callback invocation
srcu: Parallelize callback handling
kvm: Move srcu_struct fields to end of struct kvm
rcu: Fix typo in PER_RCU_NODE_PERIOD header comment
rcu: Use true/false in assignment to bool
rcu: Use bool value directly
...
Because the rcu_cblist_n_lazy_cbs() just samples the ->len_lazy counter,
and because the rcu_cblist structure is quite straightforward, it makes
sense to open-code rcu_cblist_n_lazy_cbs(p) as p->len_lazy, cutting out
a level of indirection. This commit makes this change.
Reported-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Because the rcu_cblist_n_cbs() just samples the ->len counter, and
because the rcu_cblist structure is quite straightforward, it makes
sense to open-code rcu_cblist_n_cbs(p) as p->len, cutting out a level
of indirection. This commit makes this change.
Reported-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
