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554 lines
22 KiB
554 lines
22 KiB
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_TREE_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_TREE_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 pq=1 qp=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 pq=1 qp=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 pq=1 qp=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 pq=1 qp=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 pq=1 qp=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 pq=1 qp=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 pq=1 qp=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 pq=1 qp=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 pq=1 qp=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 pq=1 qp=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 pq=1 qp=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 pq=1 qp=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 pq=1 qp=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 pq=1 qp=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 pq=1 qp=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 pq=1 qp=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 d=21872 w=0 tf=0 wd1=0 wd2=0 n=0 sc=21872 dt=21872 dl=0 dx=21872 |
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These fields are as follows: |
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o "s" is the starting sequence number. |
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o "d" is the ending sequence number. When the starting and ending |
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numbers differ, there is an expedited grace period in progress. |
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o "w" is the number of times that the sequence numbers have been |
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in danger of wrapping. |
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o "tf" is the number of times that contention has resulted in a |
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failure to begin an expedited grace period. |
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o "wd1" and "wd2" are the number of times that an attempt to |
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start an expedited grace period found that someone else had |
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completed an expedited grace period that satisfies the |
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attempted request. "Our work is done." |
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o "n" is number of times that contention was so great that |
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the request was demoted from an expedited grace period to |
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a normal grace period. |
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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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o "dt" is the number of times that we attempted to update |
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the "d" counter. |
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o "dl" is the number of times that we failed to update the "d" |
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counter. |
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o "dx" is the number of times that we succeeded in updating |
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the "d" counter. |
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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 |
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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 |
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the current expedited grace period from completing. |
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"B" This indicates that some of the blocked tasks are in |
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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 |
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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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"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 |
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expedited grace period. |
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o "nnb" is the number of tasks boosted in order to complete a |
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normal (non-expedited) grace period. When boosting a task |
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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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o "bt" is the low-order 16 bits of the value that the jiffies |
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counter will have when we next start boosting, assuming that |
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the current grace period does not end beforehand. This is |
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also in hexadecimal. |
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o "balk: nt" counts the number of times we didn't boost (in |
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other words, we balked) even though it was time to boost because |
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there were no blocked tasks to boost. This situation occurs |
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when there is one blocked task on one rcu_node structure and |
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none on some other rcu_node structure. |
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o "egt" counts the number of times we balked because although |
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there were blocked tasks, none of them were blocking the |
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current grace period, whether expedited or otherwise. |
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o "bt" counts the number of times we balked because boosting |
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had already been initiated for the current grace period. |
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o "nb" counts the number of times we balked because there |
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was at least one task blocking the current non-expedited grace |
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period that never had blocked. If it is already running, it |
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just won't help to boost its priority! |
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o "ny" counts the number of times we balked because it was |
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not yet time to start boosting. |
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o "nos" counts the number of times we balked for other |
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reasons, e.g., the grace period ended first. |
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CONFIG_TINY_RCU debugfs Files and Formats |
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These implementations of RCU provides a single debugfs file under the |
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top-level directory RCU, namely rcu/rcudata, which displays fields in |
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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 |
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rcu_bh: qlen: 0 |
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This is split into rcu_sched and rcu_bh sections. The field is as |
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follows: |
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o "qlen" is the number of RCU callbacks currently waiting either |
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for an RCU grace period or waiting to be invoked. This is the |
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only field present for rcu_sched and rcu_bh, due to the |
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short-circuiting of grace period in those two cases.
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