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e2fd9d3584
Commit #cdacbe1f91264 ("rcu: Add fastpath bypassing funnel locking")
turns out to be a pessimization at high load because it forces a tree
full of tasks to wait for an expedited grace period that they probably
do not need. This commit therefore removes this optimization.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
538 lines
22 KiB
Text
538 lines
22 KiB
Text
CONFIG_RCU_TRACE debugfs Files and Formats
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The rcutree and rcutiny implementations of RCU provide debugfs trace
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output that summarizes counters and state. This information is useful for
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debugging RCU itself, and can sometimes also help to debug abuses of RCU.
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The following sections describe the debugfs files and formats, first
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for rcutree and next for rcutiny.
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CONFIG_TREE_RCU and CONFIG_PREEMPT_RCU debugfs Files and Formats
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These implementations of RCU provide several debugfs directories under the
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top-level directory "rcu":
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rcu/rcu_bh
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rcu/rcu_preempt
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rcu/rcu_sched
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Each directory contains files for the corresponding flavor of RCU.
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Note that rcu/rcu_preempt is only present for CONFIG_PREEMPT_RCU.
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For CONFIG_TREE_RCU, the RCU flavor maps onto the RCU-sched flavor,
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so that activity for both appears in rcu/rcu_sched.
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In addition, the following file appears in the top-level directory:
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rcu/rcutorture. This file displays rcutorture test progress. The output
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of "cat rcu/rcutorture" looks as follows:
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rcutorture test sequence: 0 (test in progress)
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rcutorture update version number: 615
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The first line shows the number of rcutorture tests that have completed
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since boot. If a test is currently running, the "(test in progress)"
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string will appear as shown above. The second line shows the number of
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update cycles that the current test has started, or zero if there is
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no test in progress.
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Within each flavor directory (rcu/rcu_bh, rcu/rcu_sched, and possibly
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also rcu/rcu_preempt) the following files will be present:
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rcudata:
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Displays fields in struct rcu_data.
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rcuexp:
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Displays statistics for expedited grace periods.
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rcugp:
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Displays grace-period counters.
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rcuhier:
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Displays the struct rcu_node hierarchy.
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rcu_pending:
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Displays counts of the reasons rcu_pending() decided that RCU had
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work to do.
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rcuboost:
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Displays RCU boosting statistics. Only present if
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CONFIG_RCU_BOOST=y.
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The output of "cat rcu/rcu_preempt/rcudata" looks as follows:
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0!c=30455 g=30456 cnq=1/0:1 dt=126535/140000000000000/0 df=2002 of=4 ql=0/0 qs=N... b=10 ci=74572 nci=0 co=1131 ca=716
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1!c=30719 g=30720 cnq=1/0:0 dt=132007/140000000000000/0 df=1874 of=10 ql=0/0 qs=N... b=10 ci=123209 nci=0 co=685 ca=982
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2!c=30150 g=30151 cnq=1/1:1 dt=138537/140000000000000/0 df=1707 of=8 ql=0/0 qs=N... b=10 ci=80132 nci=0 co=1328 ca=1458
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3 c=31249 g=31250 cnq=1/1:0 dt=107255/140000000000000/0 df=1749 of=6 ql=0/450 qs=NRW. b=10 ci=151700 nci=0 co=509 ca=622
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4!c=29502 g=29503 cnq=1/0:1 dt=83647/140000000000000/0 df=965 of=5 ql=0/0 qs=N... b=10 ci=65643 nci=0 co=1373 ca=1521
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5 c=31201 g=31202 cnq=1/0:1 dt=70422/0/0 df=535 of=7 ql=0/0 qs=.... b=10 ci=58500 nci=0 co=764 ca=698
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6!c=30253 g=30254 cnq=1/0:1 dt=95363/140000000000000/0 df=780 of=5 ql=0/0 qs=N... b=10 ci=100607 nci=0 co=1414 ca=1353
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7 c=31178 g=31178 cnq=1/0:0 dt=91536/0/0 df=547 of=4 ql=0/0 qs=.... b=10 ci=109819 nci=0 co=1115 ca=969
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This file has one line per CPU, or eight for this 8-CPU system.
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The fields are as follows:
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o The number at the beginning of each line is the CPU number.
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CPUs numbers followed by an exclamation mark are offline,
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but have been online at least once since boot. There will be
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no output for CPUs that have never been online, which can be
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a good thing in the surprisingly common case where NR_CPUS is
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substantially larger than the number of actual CPUs.
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o "c" is the count of grace periods that this CPU believes have
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completed. Offlined CPUs and CPUs in dynticks idle mode may lag
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quite a ways behind, for example, CPU 4 under "rcu_sched" above,
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which has been offline through 16 RCU grace periods. It is not
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unusual to see offline CPUs lagging by thousands of grace periods.
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Note that although the grace-period number is an unsigned long,
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it is printed out as a signed long to allow more human-friendly
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representation near boot time.
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o "g" is the count of grace periods that this CPU believes have
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started. Again, offlined CPUs and CPUs in dynticks idle mode
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may lag behind. If the "c" and "g" values are equal, this CPU
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has already reported a quiescent state for the last RCU grace
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period that it is aware of, otherwise, the CPU believes that it
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owes RCU a quiescent state.
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o "pq" indicates that this CPU has passed through a quiescent state
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for the current grace period. It is possible for "pq" to be
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"1" and "c" different than "g", which indicates that although
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the CPU has passed through a quiescent state, either (1) this
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CPU has not yet reported that fact, (2) some other CPU has not
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yet reported for this grace period, or (3) both.
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o "qp" indicates that RCU still expects a quiescent state from
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this CPU. Offlined CPUs and CPUs in dyntick idle mode might
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well have qp=1, which is OK: RCU is still ignoring them.
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o "dt" is the current value of the dyntick counter that is incremented
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when entering or leaving idle, either due to a context switch or
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due to an interrupt. This number is even if the CPU is in idle
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from RCU's viewpoint and odd otherwise. The number after the
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first "/" is the interrupt nesting depth when in idle state,
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or a large number added to the interrupt-nesting depth when
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running a non-idle task. Some architectures do not accurately
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count interrupt nesting when running in non-idle kernel context,
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which can result in interesting anomalies such as negative
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interrupt-nesting levels. The number after the second "/"
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is the NMI nesting depth.
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o "df" is the number of times that some other CPU has forced a
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quiescent state on behalf of this CPU due to this CPU being in
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idle state.
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o "of" is the number of times that some other CPU has forced a
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quiescent state on behalf of this CPU due to this CPU being
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offline. In a perfect world, this might never happen, but it
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turns out that offlining and onlining a CPU can take several grace
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periods, and so there is likely to be an extended period of time
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when RCU believes that the CPU is online when it really is not.
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Please note that erring in the other direction (RCU believing a
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CPU is offline when it is really alive and kicking) is a fatal
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error, so it makes sense to err conservatively.
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o "ql" is the number of RCU callbacks currently residing on
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this CPU. The first number is the number of "lazy" callbacks
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that are known to RCU to only be freeing memory, and the number
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after the "/" is the total number of callbacks, lazy or not.
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These counters count callbacks regardless of what phase of
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grace-period processing that they are in (new, waiting for
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grace period to start, waiting for grace period to end, ready
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to invoke).
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o "qs" gives an indication of the state of the callback queue
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with four characters:
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"N" Indicates that there are callbacks queued that are not
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ready to be handled by the next grace period, and thus
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will be handled by the grace period following the next
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one.
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"R" Indicates that there are callbacks queued that are
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ready to be handled by the next grace period.
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"W" Indicates that there are callbacks queued that are
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waiting on the current grace period.
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"D" Indicates that there are callbacks queued that have
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already been handled by a prior grace period, and are
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thus waiting to be invoked. Note that callbacks in
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the process of being invoked are not counted here.
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Callbacks in the process of being invoked are those
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that have been removed from the rcu_data structures
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queues by rcu_do_batch(), but which have not yet been
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invoked.
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If there are no callbacks in a given one of the above states,
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the corresponding character is replaced by ".".
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o "b" is the batch limit for this CPU. If more than this number
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of RCU callbacks is ready to invoke, then the remainder will
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be deferred.
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o "ci" is the number of RCU callbacks that have been invoked for
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this CPU. Note that ci+nci+ql is the number of callbacks that have
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been registered in absence of CPU-hotplug activity.
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o "nci" is the number of RCU callbacks that have been offloaded from
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this CPU. This will always be zero unless the kernel was built
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with CONFIG_RCU_NOCB_CPU=y and the "rcu_nocbs=" kernel boot
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parameter was specified.
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o "co" is the number of RCU callbacks that have been orphaned due to
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this CPU going offline. These orphaned callbacks have been moved
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to an arbitrarily chosen online CPU.
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o "ca" is the number of RCU callbacks that have been adopted by this
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CPU due to other CPUs going offline. Note that ci+co-ca+ql is
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the number of RCU callbacks registered on this CPU.
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Kernels compiled with CONFIG_RCU_BOOST=y display the following from
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/debug/rcu/rcu_preempt/rcudata:
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0!c=12865 g=12866 cnq=1/0:1 dt=83113/140000000000000/0 df=288 of=11 ql=0/0 qs=N... kt=0/O ktl=944 b=10 ci=60709 nci=0 co=748 ca=871
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1 c=14407 g=14408 cnq=1/0:0 dt=100679/140000000000000/0 df=378 of=7 ql=0/119 qs=NRW. kt=0/W ktl=9b6 b=10 ci=109740 nci=0 co=589 ca=485
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2 c=14407 g=14408 cnq=1/0:0 dt=105486/0/0 df=90 of=9 ql=0/89 qs=NRW. kt=0/W ktl=c0c b=10 ci=83113 nci=0 co=533 ca=490
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3 c=14407 g=14408 cnq=1/0:0 dt=107138/0/0 df=142 of=8 ql=0/188 qs=NRW. kt=0/W ktl=b96 b=10 ci=121114 nci=0 co=426 ca=290
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4 c=14405 g=14406 cnq=1/0:1 dt=50238/0/0 df=706 of=7 ql=0/0 qs=.... kt=0/W ktl=812 b=10 ci=34929 nci=0 co=643 ca=114
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5!c=14168 g=14169 cnq=1/0:0 dt=45465/140000000000000/0 df=161 of=11 ql=0/0 qs=N... kt=0/O ktl=b4d b=10 ci=47712 nci=0 co=677 ca=722
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6 c=14404 g=14405 cnq=1/0:0 dt=59454/0/0 df=94 of=6 ql=0/0 qs=.... kt=0/W ktl=e57 b=10 ci=55597 nci=0 co=701 ca=811
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7 c=14407 g=14408 cnq=1/0:1 dt=68850/0/0 df=31 of=8 ql=0/0 qs=.... kt=0/W ktl=14bd b=10 ci=77475 nci=0 co=508 ca=1042
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This is similar to the output discussed above, but contains the following
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additional fields:
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o "kt" is the per-CPU kernel-thread state. The digit preceding
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the first slash is zero if there is no work pending and 1
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otherwise. The character between the first pair of slashes is
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as follows:
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"S" The kernel thread is stopped, in other words, all
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CPUs corresponding to this rcu_node structure are
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offline.
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"R" The kernel thread is running.
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"W" The kernel thread is waiting because there is no work
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for it to do.
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"O" The kernel thread is waiting because it has been
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forced off of its designated CPU or because its
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->cpus_allowed mask permits it to run on other than
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its designated CPU.
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"Y" The kernel thread is yielding to avoid hogging CPU.
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"?" Unknown value, indicates a bug.
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The number after the final slash is the CPU that the kthread
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is actually running on.
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This field is displayed only for CONFIG_RCU_BOOST kernels.
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o "ktl" is the low-order 16 bits (in hexadecimal) of the count of
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the number of times that this CPU's per-CPU kthread has gone
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through its loop servicing invoke_rcu_cpu_kthread() requests.
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This field is displayed only for CONFIG_RCU_BOOST kernels.
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The output of "cat rcu/rcu_preempt/rcuexp" looks as follows:
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s=21872 wd1=0 wd2=0 wd3=5 n=0 enq=0 sc=21872
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These fields are as follows:
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o "s" is the sequence number, with an odd number indicating that
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an expedited grace period is in progress.
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o "wd1", "wd2", and "wd3" are the number of times that an attempt
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to start an expedited grace period found that someone else had
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completed an expedited grace period that satisfies the attempted
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request. "Our work is done."
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o "n" is number of times that a concurrent CPU-hotplug operation
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forced a fallback to a normal grace period.
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o "enq" is the number of quiescent states still outstanding.
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o "sc" is the number of times that the attempt to start a
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new expedited grace period succeeded.
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The output of "cat rcu/rcu_preempt/rcugp" looks as follows:
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completed=31249 gpnum=31250 age=1 max=18
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These fields are taken from the rcu_state structure, and are as follows:
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o "completed" is the number of grace periods that have completed.
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It is comparable to the "c" field from rcu/rcudata in that a
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CPU whose "c" field matches the value of "completed" is aware
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that the corresponding RCU grace period has completed.
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o "gpnum" is the number of grace periods that have started. It is
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similarly comparable to the "g" field from rcu/rcudata in that
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a CPU whose "g" field matches the value of "gpnum" is aware that
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the corresponding RCU grace period has started.
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If these two fields are equal, then there is no grace period
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in progress, in other words, RCU is idle. On the other hand,
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if the two fields differ (as they are above), then an RCU grace
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period is in progress.
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o "age" is the number of jiffies that the current grace period
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has extended for, or zero if there is no grace period currently
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in effect.
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o "max" is the age in jiffies of the longest-duration grace period
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thus far.
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The output of "cat rcu/rcu_preempt/rcuhier" looks as follows:
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c=14407 g=14408 s=0 jfq=2 j=c863 nfqs=12040/nfqsng=0(12040) fqlh=1051 oqlen=0/0
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3/3 ..>. 0:7 ^0
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e/e ..>. 0:3 ^0 d/d ..>. 4:7 ^1
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The fields are as follows:
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o "c" is exactly the same as "completed" under rcu/rcu_preempt/rcugp.
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o "g" is exactly the same as "gpnum" under rcu/rcu_preempt/rcugp.
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o "s" is the current state of the force_quiescent_state()
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state machine.
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o "jfq" is the number of jiffies remaining for this grace period
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before force_quiescent_state() is invoked to help push things
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along. Note that CPUs in idle mode throughout the grace period
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will not report on their own, but rather must be check by some
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other CPU via force_quiescent_state().
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o "j" is the low-order four hex digits of the jiffies counter.
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Yes, Paul did run into a number of problems that turned out to
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be due to the jiffies counter no longer counting. Why do you ask?
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o "nfqs" is the number of calls to force_quiescent_state() since
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boot.
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o "nfqsng" is the number of useless calls to force_quiescent_state(),
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where there wasn't actually a grace period active. This can
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no longer happen due to grace-period processing being pushed
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into a kthread. The number in parentheses is the difference
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between "nfqs" and "nfqsng", or the number of times that
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force_quiescent_state() actually did some real work.
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o "fqlh" is the number of calls to force_quiescent_state() that
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exited immediately (without even being counted in nfqs above)
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due to contention on ->fqslock.
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o Each element of the form "3/3 ..>. 0:7 ^0" represents one rcu_node
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structure. Each line represents one level of the hierarchy,
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from root to leaves. It is best to think of the rcu_data
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structures as forming yet another level after the leaves.
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Note that there might be either one, two, three, or even four
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levels of rcu_node structures, depending on the relationship
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between CONFIG_RCU_FANOUT, CONFIG_RCU_FANOUT_LEAF (possibly
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adjusted using the rcu_fanout_leaf kernel boot parameter), and
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CONFIG_NR_CPUS (possibly adjusted using the nr_cpu_ids count of
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possible CPUs for the booting hardware).
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o The numbers separated by the "/" are the qsmask followed
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by the qsmaskinit. The qsmask will have one bit
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set for each entity in the next lower level that has
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not yet checked in for the current grace period ("e"
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indicating CPUs 5, 6, and 7 in the example above).
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The qsmaskinit will have one bit for each entity that is
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currently expected to check in during each grace period.
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The value of qsmaskinit is assigned to that of qsmask
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at the beginning of each grace period.
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o The characters separated by the ">" indicate the state
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of the blocked-tasks lists. A "G" preceding the ">"
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indicates that at least one task blocked in an RCU
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read-side critical section blocks the current grace
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period, while a "E" preceding the ">" indicates that
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at least one task blocked in an RCU read-side critical
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section blocks the current expedited grace period.
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A "T" character following the ">" indicates that at
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least one task is blocked within an RCU read-side
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critical section, regardless of whether any current
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grace period (expedited or normal) is inconvenienced.
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A "." character appears if the corresponding condition
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does not hold, so that "..>." indicates that no tasks
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are blocked. In contrast, "GE>T" indicates maximal
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inconvenience from blocked tasks. CONFIG_TREE_RCU
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builds of the kernel will always show "..>.".
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o The numbers separated by the ":" are the range of CPUs
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served by this struct rcu_node. This can be helpful
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in working out how the hierarchy is wired together.
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For example, the example rcu_node structure shown above
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has "0:7", indicating that it covers CPUs 0 through 7.
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o The number after the "^" indicates the bit in the
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next higher level rcu_node structure that this rcu_node
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structure corresponds to. For example, the "d/d ..>. 4:7
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^1" has a "1" in this position, indicating that it
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corresponds to the "1" bit in the "3" shown in the
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"3/3 ..>. 0:7 ^0" entry on the next level up.
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The output of "cat rcu/rcu_sched/rcu_pending" looks as follows:
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0!np=26111 qsp=29 rpq=5386 cbr=1 cng=570 gpc=3674 gps=577 nn=15903 ndw=0
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1!np=28913 qsp=35 rpq=6097 cbr=1 cng=448 gpc=3700 gps=554 nn=18113 ndw=0
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2!np=32740 qsp=37 rpq=6202 cbr=0 cng=476 gpc=4627 gps=546 nn=20889 ndw=0
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3 np=23679 qsp=22 rpq=5044 cbr=1 cng=415 gpc=3403 gps=347 nn=14469 ndw=0
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4!np=30714 qsp=4 rpq=5574 cbr=0 cng=528 gpc=3931 gps=639 nn=20042 ndw=0
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5 np=28910 qsp=2 rpq=5246 cbr=0 cng=428 gpc=4105 gps=709 nn=18422 ndw=0
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6!np=38648 qsp=5 rpq=7076 cbr=0 cng=840 gpc=4072 gps=961 nn=25699 ndw=0
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7 np=37275 qsp=2 rpq=6873 cbr=0 cng=868 gpc=3416 gps=971 nn=25147 ndw=0
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The fields are as follows:
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o The leading number is the CPU number, with "!" indicating
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an offline CPU.
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o "np" is the number of times that __rcu_pending() has been invoked
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for the corresponding flavor of RCU.
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o "qsp" is the number of times that the RCU was waiting for a
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quiescent state from this CPU.
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o "rpq" is the number of times that the CPU had passed through
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a quiescent state, but not yet reported it to RCU.
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o "cbr" is the number of times that this CPU had RCU callbacks
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that had passed through a grace period, and were thus ready
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to be invoked.
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o "cng" is the number of times that this CPU needed another
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grace period while RCU was idle.
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o "gpc" is the number of times that an old grace period had
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completed, but this CPU was not yet aware of it.
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o "gps" is the number of times that a new grace period had started,
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but this CPU was not yet aware of it.
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o "ndw" is the number of times that a wakeup of an rcuo
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callback-offload kthread had to be deferred in order to avoid
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deadlock.
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o "nn" is the number of times that this CPU needed nothing.
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The output of "cat rcu/rcuboost" looks as follows:
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0:3 tasks=.... kt=W ntb=0 neb=0 nnb=0 j=c864 bt=c894
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balk: nt=0 egt=4695 bt=0 nb=0 ny=56 nos=0
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4:7 tasks=.... kt=W ntb=0 neb=0 nnb=0 j=c864 bt=c894
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balk: nt=0 egt=6541 bt=0 nb=0 ny=126 nos=0
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This information is output only for rcu_preempt. Each two-line entry
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corresponds to a leaf rcu_node structure. The fields are as follows:
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o "n:m" is the CPU-number range for the corresponding two-line
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entry. In the sample output above, the first entry covers
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CPUs zero through three and the second entry covers CPUs four
|
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through seven.
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o "tasks=TNEB" gives the state of the various segments of the
|
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rnp->blocked_tasks list:
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"T" This indicates that there are some tasks that blocked
|
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while running on one of the corresponding CPUs while
|
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in an RCU read-side critical section.
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"N" This indicates that some of the blocked tasks are preventing
|
|
the current normal (non-expedited) grace period from
|
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completing.
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"E" This indicates that some of the blocked tasks are preventing
|
|
the current expedited grace period from completing.
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"B" This indicates that some of the blocked tasks are in
|
|
need of RCU priority boosting.
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Each character is replaced with "." if the corresponding
|
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condition does not hold.
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o "kt" is the state of the RCU priority-boosting kernel
|
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thread associated with the corresponding rcu_node structure.
|
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The state can be one of the following:
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"S" The kernel thread is stopped, in other words, all
|
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CPUs corresponding to this rcu_node structure are
|
|
offline.
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"R" The kernel thread is running.
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"W" The kernel thread is waiting because there is no work
|
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for it to do.
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"Y" The kernel thread is yielding to avoid hogging CPU.
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"?" Unknown value, indicates a bug.
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o "ntb" is the number of tasks boosted.
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o "neb" is the number of tasks boosted in order to complete an
|
|
expedited grace period.
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|
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o "nnb" is the number of tasks boosted in order to complete a
|
|
normal (non-expedited) grace period. When boosting a task
|
|
that was blocking both an expedited and a normal grace period,
|
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it is counted against the expedited total above.
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o "j" is the low-order 16 bits of the jiffies counter in
|
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hexadecimal.
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|
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o "bt" is the low-order 16 bits of the value that the jiffies
|
|
counter will have when we next start boosting, assuming that
|
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the current grace period does not end beforehand. This is
|
|
also in hexadecimal.
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|
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o "balk: nt" counts the number of times we didn't boost (in
|
|
other words, we balked) even though it was time to boost because
|
|
there were no blocked tasks to boost. This situation occurs
|
|
when there is one blocked task on one rcu_node structure and
|
|
none on some other rcu_node structure.
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|
|
o "egt" counts the number of times we balked because although
|
|
there were blocked tasks, none of them were blocking the
|
|
current grace period, whether expedited or otherwise.
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|
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o "bt" counts the number of times we balked because boosting
|
|
had already been initiated for the current grace period.
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|
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o "nb" counts the number of times we balked because there
|
|
was at least one task blocking the current non-expedited grace
|
|
period that never had blocked. If it is already running, it
|
|
just won't help to boost its priority!
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|
o "ny" counts the number of times we balked because it was
|
|
not yet time to start boosting.
|
|
|
|
o "nos" counts the number of times we balked for other
|
|
reasons, e.g., the grace period ended first.
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|
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|
|
CONFIG_TINY_RCU debugfs Files and Formats
|
|
|
|
These implementations of RCU provides a single debugfs file under the
|
|
top-level directory RCU, namely rcu/rcudata, which displays fields in
|
|
rcu_bh_ctrlblk and rcu_sched_ctrlblk.
|
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|
|
The output of "cat rcu/rcudata" is as follows:
|
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|
|
rcu_sched: qlen: 0
|
|
rcu_bh: qlen: 0
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|
|
This is split into rcu_sched and rcu_bh sections. The field is as
|
|
follows:
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|
|
o "qlen" is the number of RCU callbacks currently waiting either
|
|
for an RCU grace period or waiting to be invoked. This is the
|
|
only field present for rcu_sched and rcu_bh, due to the
|
|
short-circuiting of grace period in those two cases.
|