linux-stable/Documentation/RCU/stallwarn.rst

366 lines
17 KiB
ReStructuredText
Raw Normal View History

.. SPDX-License-Identifier: GPL-2.0
==============================
Using RCU's CPU Stall Detector
==============================
This document first discusses what sorts of issues RCU's CPU stall
detector can locate, and then discusses kernel parameters and Kconfig
options that can be used to fine-tune the detector's operation. Finally,
this document explains the stall detector's "splat" format.
What Causes RCU CPU Stall Warnings?
===================================
So your kernel printed an RCU CPU stall warning. The next question is
"What caused it?" The following problems can result in RCU CPU stall
warnings:
- A CPU looping in an RCU read-side critical section.
- A CPU looping with interrupts disabled.
- A CPU looping with preemption disabled.
- A CPU looping with bottom halves disabled.
- For !CONFIG_PREEMPTION kernels, a CPU looping anywhere in the kernel
without invoking schedule(). If the looping in the kernel is
really expected and desirable behavior, you might need to add
some calls to cond_resched().
- Booting Linux using a console connection that is too slow to
keep up with the boot-time console-message rate. For example,
a 115Kbaud serial console can be *way* too slow to keep up
with boot-time message rates, and will frequently result in
RCU CPU stall warning messages. Especially if you have added
debug printk()s.
- Anything that prevents RCU's grace-period kthreads from running.
This can result in the "All QSes seen" console-log message.
This message will include information on when the kthread last
ran and how often it should be expected to run. It can also
result in the ``rcu_.*kthread starved for`` console-log message,
which will include additional debugging information.
- A CPU-bound real-time task in a CONFIG_PREEMPTION kernel, which might
happen to preempt a low-priority task in the middle of an RCU
read-side critical section. This is especially damaging if
that low-priority task is not permitted to run on any other CPU,
in which case the next RCU grace period can never complete, which
will eventually cause the system to run out of memory and hang.
While the system is in the process of running itself out of
memory, you might see stall-warning messages.
- A CPU-bound real-time task in a CONFIG_PREEMPT_RT kernel that
is running at a higher priority than the RCU softirq threads.
This will prevent RCU callbacks from ever being invoked,
and in a CONFIG_PREEMPT_RCU kernel will further prevent
RCU grace periods from ever completing. Either way, the
system will eventually run out of memory and hang. In the
CONFIG_PREEMPT_RCU case, you might see stall-warning
messages.
You can use the rcutree.kthread_prio kernel boot parameter to
increase the scheduling priority of RCU's kthreads, which can
help avoid this problem. However, please note that doing this
can increase your system's context-switch rate and thus degrade
performance.
- A periodic interrupt whose handler takes longer than the time
interval between successive pairs of interrupts. This can
prevent RCU's kthreads and softirq handlers from running.
Note that certain high-overhead debugging options, for example
the function_graph tracer, can result in interrupt handler taking
considerably longer than normal, which can in turn result in
RCU CPU stall warnings.
- Testing a workload on a fast system, tuning the stall-warning
timeout down to just barely avoid RCU CPU stall warnings, and then
running the same workload with the same stall-warning timeout on a
slow system. Note that thermal throttling and on-demand governors
can cause a single system to be sometimes fast and sometimes slow!
- A hardware or software issue shuts off the scheduler-clock
interrupt on a CPU that is not in dyntick-idle mode. This
problem really has happened, and seems to be most likely to
result in RCU CPU stall warnings for CONFIG_NO_HZ_COMMON=n kernels.
- A hardware or software issue that prevents time-based wakeups
from occurring. These issues can range from misconfigured or
buggy timer hardware through bugs in the interrupt or exception
path (whether hardware, firmware, or software) through bugs
in Linux's timer subsystem through bugs in the scheduler, and,
rcu: Check and report missed fqs timer wakeup on RCU stall For a new grace period request, the RCU GP kthread transitions through following states: a. [RCU_GP_WAIT_GPS] -> [RCU_GP_DONE_GPS] The RCU_GP_WAIT_GPS state is where the GP kthread waits for a request for a new GP. Once it receives a request (for example, when a new RCU callback is queued), the GP kthread transitions to RCU_GP_DONE_GPS. b. [RCU_GP_DONE_GPS] -> [RCU_GP_ONOFF] Grace period initialization starts in rcu_gp_init(), which records the start of new GP in rcu_state.gp_seq and transitions to RCU_GP_ONOFF. c. [RCU_GP_ONOFF] -> [RCU_GP_INIT] The purpose of the RCU_GP_ONOFF state is to apply the online/offline information that was buffered for any CPUs that recently came online or went offline. This state is maintained in per-leaf rcu_node bitmasks, with the buffered state in ->qsmaskinitnext and the state for the upcoming GP in ->qsmaskinit. At the end of this RCU_GP_ONOFF state, each bit in ->qsmaskinit will correspond to a CPU that must pass through a quiescent state before the upcoming grace period is allowed to complete. However, a leaf rcu_node structure with an all-zeroes ->qsmaskinit cannot necessarily be ignored. In preemptible RCU, there might well be tasks still in RCU read-side critical sections that were first preempted while running on one of the CPUs managed by this structure. Such tasks will be queued on this structure's ->blkd_tasks list. Only after this list fully drains can this leaf rcu_node structure be ignored, and even then only if none of its CPUs have come back online in the meantime. Once that happens, the ->qsmaskinit masks further up the tree will be updated to exclude this leaf rcu_node structure. Once the ->qsmaskinitnext and ->qsmaskinit fields have been updated as needed, the GP kthread transitions to RCU_GP_INIT. d. [RCU_GP_INIT] -> [RCU_GP_WAIT_FQS] The purpose of the RCU_GP_INIT state is to copy each ->qsmaskinit to the ->qsmask field within each rcu_node structure. This copying is done breadth-first from the root to the leaves. Why not just copy directly from ->qsmaskinitnext to ->qsmask? Because the ->qsmaskinitnext masks can change in the meantime as additional CPUs come online or go offline. Such changes would result in inconsistencies in the ->qsmask fields up and down the tree, which could in turn result in too-short grace periods or grace-period hangs. These issues are avoided by snapshotting the leaf rcu_node structures' ->qsmaskinitnext fields into their ->qsmaskinit counterparts, generating a consistent set of ->qsmaskinit fields throughout the tree, and only then copying these consistent ->qsmaskinit fields to their ->qsmask counterparts. Once this initialization step is complete, the GP kthread transitions to RCU_GP_WAIT_FQS, where it waits to do a force-quiescent-state scan on the one hand or for the end of the grace period on the other. e. [RCU_GP_WAIT_FQS] -> [RCU_GP_DOING_FQS] The RCU_GP_WAIT_FQS state waits for one of three things: (1) An explicit request to do a force-quiescent-state scan, (2) The end of the grace period, or (3) A short interval of time, after which it will do a force-quiescent-state (FQS) scan. The explicit request can come from rcutorture or from any CPU that has too many RCU callbacks queued (see the qhimark kernel parameter and the RCU_GP_FLAG_OVLD flag). The aforementioned "short period of time" is specified by the jiffies_till_first_fqs boot parameter for a given grace period's first FQS scan and by the jiffies_till_next_fqs for later FQS scans. Either way, once the wait is over, the GP kthread transitions to RCU_GP_DOING_FQS. f. [RCU_GP_DOING_FQS] -> [RCU_GP_CLEANUP] The RCU_GP_DOING_FQS state performs an FQS scan. Each such scan carries out two functions for any CPU whose bit is still set in its leaf rcu_node structure's ->qsmask field, that is, for any CPU that has not yet reported a quiescent state for the current grace period: i. Report quiescent states on behalf of CPUs that have been observed to be idle (from an RCU perspective) since the beginning of the grace period. ii. If the current grace period is too old, take various actions to encourage holdout CPUs to pass through quiescent states, including enlisting the aid of any calls to cond_resched() and might_sleep(), and even including IPIing the holdout CPUs. These checks are skipped for any leaf rcu_node structure with a all-zero ->qsmask field, however such structures are subject to RCU priority boosting if there are tasks on a given structure blocking the current grace period. The end of the grace period is detected when the root rcu_node structure's ->qsmask is zero and when there are no longer any preempted tasks blocking the current grace period. (No, this last check is not redundant. To see this, consider an rcu_node tree having exactly one structure that serves as both root and leaf.) Once the end of the grace period is detected, the GP kthread transitions to RCU_GP_CLEANUP. g. [RCU_GP_CLEANUP] -> [RCU_GP_CLEANED] The RCU_GP_CLEANUP state marks the end of grace period by updating the rcu_state structure's ->gp_seq field and also all rcu_node structures' ->gp_seq field. As before, the rcu_node tree is traversed in breadth first order. Once this update is complete, the GP kthread transitions to the RCU_GP_CLEANED state. i. [RCU_GP_CLEANED] -> [RCU_GP_INIT] Once in the RCU_GP_CLEANED state, the GP kthread immediately transitions into the RCU_GP_INIT state. j. The role of timers. If there is at least one idle CPU, and if timers are not firing, the transition from RCU_GP_DOING_FQS to RCU_GP_CLEANUP will never happen. Timers can fail to fire for a number of reasons, including issues in timer configuration, issues in the timer framework, and failure to handle softirqs (for example, when there is a storm of interrupts). Whatever the reason, if the timers fail to fire, the GP kthread will never be awakened, resulting in RCU CPU stall warnings and eventually in OOM. However, an RCU CPU stall warning has a large number of potential causes, as documented in Documentation/RCU/stallwarn.rst. This commit therefore adds analysis to the RCU CPU stall-warning code to emit an additional message if the cause of the stall is likely to be timer failure. Signed-off-by: Neeraj Upadhyay <neeraju@codeaurora.org> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
2020-11-16 16:06:00 +00:00
yes, even including bugs in RCU itself. It can also result in
the ``rcu_.*timer wakeup didn't happen for`` console-log message,
which will include additional debugging information.
- A low-level kernel issue that either fails to invoke one of the
variants of rcu_user_enter(), rcu_user_exit(), rcu_idle_enter(),
rcu_idle_exit(), rcu_irq_enter(), or rcu_irq_exit() on the one
hand, or that invokes one of them too many times on the other.
Historically, the most frequent issue has been an omission
of either irq_enter() or irq_exit(), which in turn invoke
rcu_irq_enter() or rcu_irq_exit(), respectively. Building your
kernel with CONFIG_RCU_EQS_DEBUG=y can help track down these types
of issues, which sometimes arise in architecture-specific code.
- A bug in the RCU implementation.
- A hardware failure. This is quite unlikely, but has occurred
at least once in real life. A CPU failed in a running system,
becoming unresponsive, but not causing an immediate crash.
This resulted in a series of RCU CPU stall warnings, eventually
leading the realization that the CPU had failed.
The RCU, RCU-sched, and RCU-tasks implementations have CPU stall warning.
Note that SRCU does *not* have CPU stall warnings. Please note that
RCU only detects CPU stalls when there is a grace period in progress.
No grace period, no CPU stall warnings.
To diagnose the cause of the stall, inspect the stack traces.
The offending function will usually be near the top of the stack.
If you have a series of stall warnings from a single extended stall,
comparing the stack traces can often help determine where the stall
is occurring, which will usually be in the function nearest the top of
that portion of the stack which remains the same from trace to trace.
If you can reliably trigger the stall, ftrace can be quite helpful.
RCU bugs can often be debugged with the help of CONFIG_RCU_TRACE
and with RCU's event tracing. For information on RCU's event tracing,
see include/trace/events/rcu.h.
Fine-Tuning the RCU CPU Stall Detector
======================================
The rcuupdate.rcu_cpu_stall_suppress module parameter disables RCU's
CPU stall detector, which detects conditions that unduly delay RCU grace
periods. This module parameter enables CPU stall detection by default,
but may be overridden via boot-time parameter or at runtime via sysfs.
The stall detector's idea of what constitutes "unduly delayed" is
controlled by a set of kernel configuration variables and cpp macros:
CONFIG_RCU_CPU_STALL_TIMEOUT
----------------------------
This kernel configuration parameter defines the period of time
that RCU will wait from the beginning of a grace period until it
issues an RCU CPU stall warning. This time period is normally
21 seconds.
This configuration parameter may be changed at runtime via the
/sys/module/rcupdate/parameters/rcu_cpu_stall_timeout, however
this parameter is checked only at the beginning of a cycle.
So if you are 10 seconds into a 40-second stall, setting this
sysfs parameter to (say) five will shorten the timeout for the
*next* stall, or the following warning for the current stall
(assuming the stall lasts long enough). It will not affect the
timing of the next warning for the current stall.
Stall-warning messages may be enabled and disabled completely via
/sys/module/rcupdate/parameters/rcu_cpu_stall_suppress.
RCU_STALL_DELAY_DELTA
---------------------
Although the lockdep facility is extremely useful, it does add
some overhead. Therefore, under CONFIG_PROVE_RCU, the
RCU_STALL_DELAY_DELTA macro allows five extra seconds before
giving an RCU CPU stall warning message. (This is a cpp
macro, not a kernel configuration parameter.)
RCU_STALL_RAT_DELAY
-------------------
The CPU stall detector tries to make the offending CPU print its
own warnings, as this often gives better-quality stack traces.
However, if the offending CPU does not detect its own stall in
the number of jiffies specified by RCU_STALL_RAT_DELAY, then
some other CPU will complain. This delay is normally set to
two jiffies. (This is a cpp macro, not a kernel configuration
parameter.)
rcupdate.rcu_task_stall_timeout
-------------------------------
This boot/sysfs parameter controls the RCU-tasks stall warning
interval. A value of zero or less suppresses RCU-tasks stall
warnings. A positive value sets the stall-warning interval
in seconds. An RCU-tasks stall warning starts with the line:
INFO: rcu_tasks detected stalls on tasks:
And continues with the output of sched_show_task() for each
task stalling the current RCU-tasks grace period.
Interpreting RCU's CPU Stall-Detector "Splats"
==============================================
For non-RCU-tasks flavors of RCU, when a CPU detects that some other
CPU is stalling, it will print a message similar to the following::
INFO: rcu_sched detected stalls on CPUs/tasks:
2-...: (3 GPs behind) idle=06c/0/0 softirq=1453/1455 fqs=0
16-...: (0 ticks this GP) idle=81c/0/0 softirq=764/764 fqs=0
(detected by 32, t=2603 jiffies, g=7075, q=625)
This message indicates that CPU 32 detected that CPUs 2 and 16 were both
causing stalls, and that the stall was affecting RCU-sched. This message
will normally be followed by stack dumps for each CPU. Please note that
PREEMPT_RCU builds can be stalled by tasks as well as by CPUs, and that
the tasks will be indicated by PID, for example, "P3421". It is even
possible for an rcu_state stall to be caused by both CPUs *and* tasks,
in which case the offending CPUs and tasks will all be called out in the list.
In some cases, CPUs will detect themselves stalling, which will result
in a self-detected stall.
CPU 2's "(3 GPs behind)" indicates that this CPU has not interacted with
the RCU core for the past three grace periods. In contrast, CPU 16's "(0
ticks this GP)" indicates that this CPU has not taken any scheduling-clock
interrupts during the current stalled grace period.
The "idle=" portion of the message prints the dyntick-idle state.
The hex number before the first "/" is the low-order 12 bits of the
dynticks counter, which will have an even-numbered value if the CPU
is in dyntick-idle mode and an odd-numbered value otherwise. The hex
number between the two "/"s is the value of the nesting, which will be
a small non-negative number if in the idle loop (as shown above) and a
very large positive number otherwise.
The "softirq=" portion of the message tracks the number of RCU softirq
handlers that the stalled CPU has executed. The number before the "/"
is the number that had executed since boot at the time that this CPU
last noted the beginning of a grace period, which might be the current
(stalled) grace period, or it might be some earlier grace period (for
example, if the CPU might have been in dyntick-idle mode for an extended
time period). The number after the "/" is the number that have executed
since boot until the current time. If this latter number stays constant
across repeated stall-warning messages, it is possible that RCU's softirq
handlers are no longer able to execute on this CPU. This can happen if
the stalled CPU is spinning with interrupts are disabled, or, in -rt
kernels, if a high-priority process is starving RCU's softirq handler.
The "fqs=" shows the number of force-quiescent-state idle/offline
detection passes that the grace-period kthread has made across this
CPU since the last time that this CPU noted the beginning of a grace
period.
The "detected by" line indicates which CPU detected the stall (in this
case, CPU 32), how many jiffies have elapsed since the start of the grace
period (in this case 2603), the grace-period sequence number (7075), and
an estimate of the total number of RCU callbacks queued across all CPUs
(625 in this case).
If the grace period ends just as the stall warning starts printing,
there will be a spurious stall-warning message, which will include
the following::
INFO: Stall ended before state dump start
This is rare, but does happen from time to time in real life. It is also
possible for a zero-jiffy stall to be flagged in this case, depending
on how the stall warning and the grace-period initialization happen to
interact. Please note that it is not possible to entirely eliminate this
sort of false positive without resorting to things like stop_machine(),
which is overkill for this sort of problem.
If all CPUs and tasks have passed through quiescent states, but the
grace period has nevertheless failed to end, the stall-warning splat
will include something like the following::
All QSes seen, last rcu_preempt kthread activity 23807 (4297905177-4297881370), jiffies_till_next_fqs=3, root ->qsmask 0x0
The "23807" indicates that it has been more than 23 thousand jiffies
since the grace-period kthread ran. The "jiffies_till_next_fqs"
indicates how frequently that kthread should run, giving the number
of jiffies between force-quiescent-state scans, in this case three,
which is way less than 23807. Finally, the root rcu_node structure's
->qsmask field is printed, which will normally be zero.
If the relevant grace-period kthread has been unable to run prior to
the stall warning, as was the case in the "All QSes seen" line above,
the following additional line is printed::
rcu_sched kthread starved for 23807 jiffies! g7075 f0x0 RCU_GP_WAIT_FQS(3) ->state=0x1 ->cpu=5
Unless rcu_sched kthread gets sufficient CPU time, OOM is now expected behavior.
Starving the grace-period kthreads of CPU time can of course result
in RCU CPU stall warnings even when all CPUs and tasks have passed
through the required quiescent states. The "g" number shows the current
grace-period sequence number, the "f" precedes the ->gp_flags command
to the grace-period kthread, the "RCU_GP_WAIT_FQS" indicates that the
kthread is waiting for a short timeout, the "state" precedes value of the
task_struct ->state field, and the "cpu" indicates that the grace-period
kthread last ran on CPU 5.
rcu: Check and report missed fqs timer wakeup on RCU stall For a new grace period request, the RCU GP kthread transitions through following states: a. [RCU_GP_WAIT_GPS] -> [RCU_GP_DONE_GPS] The RCU_GP_WAIT_GPS state is where the GP kthread waits for a request for a new GP. Once it receives a request (for example, when a new RCU callback is queued), the GP kthread transitions to RCU_GP_DONE_GPS. b. [RCU_GP_DONE_GPS] -> [RCU_GP_ONOFF] Grace period initialization starts in rcu_gp_init(), which records the start of new GP in rcu_state.gp_seq and transitions to RCU_GP_ONOFF. c. [RCU_GP_ONOFF] -> [RCU_GP_INIT] The purpose of the RCU_GP_ONOFF state is to apply the online/offline information that was buffered for any CPUs that recently came online or went offline. This state is maintained in per-leaf rcu_node bitmasks, with the buffered state in ->qsmaskinitnext and the state for the upcoming GP in ->qsmaskinit. At the end of this RCU_GP_ONOFF state, each bit in ->qsmaskinit will correspond to a CPU that must pass through a quiescent state before the upcoming grace period is allowed to complete. However, a leaf rcu_node structure with an all-zeroes ->qsmaskinit cannot necessarily be ignored. In preemptible RCU, there might well be tasks still in RCU read-side critical sections that were first preempted while running on one of the CPUs managed by this structure. Such tasks will be queued on this structure's ->blkd_tasks list. Only after this list fully drains can this leaf rcu_node structure be ignored, and even then only if none of its CPUs have come back online in the meantime. Once that happens, the ->qsmaskinit masks further up the tree will be updated to exclude this leaf rcu_node structure. Once the ->qsmaskinitnext and ->qsmaskinit fields have been updated as needed, the GP kthread transitions to RCU_GP_INIT. d. [RCU_GP_INIT] -> [RCU_GP_WAIT_FQS] The purpose of the RCU_GP_INIT state is to copy each ->qsmaskinit to the ->qsmask field within each rcu_node structure. This copying is done breadth-first from the root to the leaves. Why not just copy directly from ->qsmaskinitnext to ->qsmask? Because the ->qsmaskinitnext masks can change in the meantime as additional CPUs come online or go offline. Such changes would result in inconsistencies in the ->qsmask fields up and down the tree, which could in turn result in too-short grace periods or grace-period hangs. These issues are avoided by snapshotting the leaf rcu_node structures' ->qsmaskinitnext fields into their ->qsmaskinit counterparts, generating a consistent set of ->qsmaskinit fields throughout the tree, and only then copying these consistent ->qsmaskinit fields to their ->qsmask counterparts. Once this initialization step is complete, the GP kthread transitions to RCU_GP_WAIT_FQS, where it waits to do a force-quiescent-state scan on the one hand or for the end of the grace period on the other. e. [RCU_GP_WAIT_FQS] -> [RCU_GP_DOING_FQS] The RCU_GP_WAIT_FQS state waits for one of three things: (1) An explicit request to do a force-quiescent-state scan, (2) The end of the grace period, or (3) A short interval of time, after which it will do a force-quiescent-state (FQS) scan. The explicit request can come from rcutorture or from any CPU that has too many RCU callbacks queued (see the qhimark kernel parameter and the RCU_GP_FLAG_OVLD flag). The aforementioned "short period of time" is specified by the jiffies_till_first_fqs boot parameter for a given grace period's first FQS scan and by the jiffies_till_next_fqs for later FQS scans. Either way, once the wait is over, the GP kthread transitions to RCU_GP_DOING_FQS. f. [RCU_GP_DOING_FQS] -> [RCU_GP_CLEANUP] The RCU_GP_DOING_FQS state performs an FQS scan. Each such scan carries out two functions for any CPU whose bit is still set in its leaf rcu_node structure's ->qsmask field, that is, for any CPU that has not yet reported a quiescent state for the current grace period: i. Report quiescent states on behalf of CPUs that have been observed to be idle (from an RCU perspective) since the beginning of the grace period. ii. If the current grace period is too old, take various actions to encourage holdout CPUs to pass through quiescent states, including enlisting the aid of any calls to cond_resched() and might_sleep(), and even including IPIing the holdout CPUs. These checks are skipped for any leaf rcu_node structure with a all-zero ->qsmask field, however such structures are subject to RCU priority boosting if there are tasks on a given structure blocking the current grace period. The end of the grace period is detected when the root rcu_node structure's ->qsmask is zero and when there are no longer any preempted tasks blocking the current grace period. (No, this last check is not redundant. To see this, consider an rcu_node tree having exactly one structure that serves as both root and leaf.) Once the end of the grace period is detected, the GP kthread transitions to RCU_GP_CLEANUP. g. [RCU_GP_CLEANUP] -> [RCU_GP_CLEANED] The RCU_GP_CLEANUP state marks the end of grace period by updating the rcu_state structure's ->gp_seq field and also all rcu_node structures' ->gp_seq field. As before, the rcu_node tree is traversed in breadth first order. Once this update is complete, the GP kthread transitions to the RCU_GP_CLEANED state. i. [RCU_GP_CLEANED] -> [RCU_GP_INIT] Once in the RCU_GP_CLEANED state, the GP kthread immediately transitions into the RCU_GP_INIT state. j. The role of timers. If there is at least one idle CPU, and if timers are not firing, the transition from RCU_GP_DOING_FQS to RCU_GP_CLEANUP will never happen. Timers can fail to fire for a number of reasons, including issues in timer configuration, issues in the timer framework, and failure to handle softirqs (for example, when there is a storm of interrupts). Whatever the reason, if the timers fail to fire, the GP kthread will never be awakened, resulting in RCU CPU stall warnings and eventually in OOM. However, an RCU CPU stall warning has a large number of potential causes, as documented in Documentation/RCU/stallwarn.rst. This commit therefore adds analysis to the RCU CPU stall-warning code to emit an additional message if the cause of the stall is likely to be timer failure. Signed-off-by: Neeraj Upadhyay <neeraju@codeaurora.org> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
2020-11-16 16:06:00 +00:00
If the relevant grace-period kthread does not wake from FQS wait in a
reasonable time, then the following additional line is printed::
kthread timer wakeup didn't happen for 23804 jiffies! g7076 f0x0 RCU_GP_WAIT_FQS(5) ->state=0x402
The "23804" indicates that kthread's timer expired more than 23 thousand
jiffies ago. The rest of the line has meaning similar to the kthread
starvation case.
Additionally, the following line is printed::
Possible timer handling issue on cpu=4 timer-softirq=11142
Here "cpu" indicates that the grace-period kthread last ran on CPU 4,
where it queued the fqs timer. The number following the "timer-softirq"
is the current ``TIMER_SOFTIRQ`` count on cpu 4. If this value does not
change on successive RCU CPU stall warnings, there is further reason to
suspect a timer problem.
These messages are usually followed by stack dumps of the CPUs and tasks
involved in the stall. These stack traces can help you locate the cause
of the stall, keeping in mind that the CPU detecting the stall will have
an interrupt frame that is mainly devoted to detecting the stall.
Multiple Warnings From One Stall
================================
If a stall lasts long enough, multiple stall-warning messages will
be printed for it. The second and subsequent messages are printed at
longer intervals, so that the time between (say) the first and second
message will be about three times the interval between the beginning
of the stall and the first message. It can be helpful to compare the
stack dumps for the different messages for the same stalled grace period.
Stall Warnings for Expedited Grace Periods
==========================================
If an expedited grace period detects a stall, it will place a message
like the following in dmesg::
INFO: rcu_sched detected expedited stalls on CPUs/tasks: { 7-... } 21119 jiffies s: 73 root: 0x2/.
This indicates that CPU 7 has failed to respond to a reschedule IPI.
The three periods (".") following the CPU number indicate that the CPU
is online (otherwise the first period would instead have been "O"),
that the CPU was online at the beginning of the expedited grace period
(otherwise the second period would have instead been "o"), and that
the CPU has been online at least once since boot (otherwise, the third
period would instead have been "N"). The number before the "jiffies"
indicates that the expedited grace period has been going on for 21,119
jiffies. The number following the "s:" indicates that the expedited
grace-period sequence counter is 73. The fact that this last value is
odd indicates that an expedited grace period is in flight. The number
following "root:" is a bitmask that indicates which children of the root
rcu_node structure correspond to CPUs and/or tasks that are blocking the
current expedited grace period. If the tree had more than one level,
additional hex numbers would be printed for the states of the other
rcu_node structures in the tree.
As with normal grace periods, PREEMPT_RCU builds can be stalled by
tasks as well as by CPUs, and that the tasks will be indicated by PID,
for example, "P3421".
It is entirely possible to see stall warnings from normal and from
expedited grace periods at about the same time during the same run.