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Diffstat (limited to 'Documentation/RCU')
| -rw-r--r-- | Documentation/RCU/trace.txt | 231 |
1 files changed, 33 insertions, 198 deletions
diff --git a/Documentation/RCU/trace.txt b/Documentation/RCU/trace.txt index 187bbf10c92..c1a95507d10 100644 --- a/Documentation/RCU/trace.txt +++ b/Documentation/RCU/trace.txt @@ -1,185 +1,10 @@ CONFIG_RCU_TRACE debugfs Files and Formats -The rcupreempt and rcutree implementations of RCU provide debugfs trace -output that summarizes counters and state. This information is useful for -debugging RCU itself, and can sometimes also help to debug abuses of RCU. -Note that the rcuclassic implementation of RCU does not provide debugfs -trace output. - -The following sections describe the debugfs files and formats for -preemptable RCU (rcupreempt) and hierarchical RCU (rcutree). - - -Preemptable RCU debugfs Files and Formats - -This implementation of RCU provides three debugfs files under the -top-level directory RCU: rcu/rcuctrs (which displays the per-CPU -counters used by preemptable RCU) rcu/rcugp (which displays grace-period -counters), and rcu/rcustats (which internal counters for debugging RCU). - -The output of "cat rcu/rcuctrs" looks as follows: - -CPU last cur F M - 0 5 -5 0 0 - 1 -1 0 0 0 - 2 0 1 0 0 - 3 0 1 0 0 - 4 0 1 0 0 - 5 0 1 0 0 - 6 0 2 0 0 - 7 0 -1 0 0 - 8 0 1 0 0 -ggp = 26226, state = waitzero - -The per-CPU fields are as follows: - -o "CPU" gives the CPU number. Offline CPUs are not displayed. - -o "last" gives the value of the counter that is being decremented - for the current grace period phase. In the example above, - the counters sum to 4, indicating that there are still four - RCU read-side critical sections still running that started - before the last counter flip. - -o "cur" gives the value of the counter that is currently being - both incremented (by rcu_read_lock()) and decremented (by - rcu_read_unlock()). In the example above, the counters sum to - 1, indicating that there is only one RCU read-side critical section - still running that started after the last counter flip. - -o "F" indicates whether RCU is waiting for this CPU to acknowledge - a counter flip. In the above example, RCU is not waiting on any, - which is consistent with the state being "waitzero" rather than - "waitack". - -o "M" indicates whether RCU is waiting for this CPU to execute a - memory barrier. In the above example, RCU is not waiting on any, - which is consistent with the state being "waitzero" rather than - "waitmb". - -o "ggp" is the global grace-period counter. - -o "state" is the RCU state, which can be one of the following: - - o "idle": there is no grace period in progress. - - o "waitack": RCU just incremented the global grace-period - counter, which has the effect of reversing the roles of - the "last" and "cur" counters above, and is waiting for - all the CPUs to acknowledge the flip. Once the flip has - been acknowledged, CPUs will no longer be incrementing - what are now the "last" counters, so that their sum will - decrease monotonically down to zero. - - o "waitzero": RCU is waiting for the sum of the "last" counters - to decrease to zero. - - o "waitmb": RCU is waiting for each CPU to execute a memory - barrier, which ensures that instructions from a given CPU's - last RCU read-side critical section cannot be reordered - with instructions following the memory-barrier instruction. - -The output of "cat rcu/rcugp" looks as follows: - -oldggp=48870 newggp=48873 - -Note that reading from this file provokes a synchronize_rcu(). The -"oldggp" value is that of "ggp" from rcu/rcuctrs above, taken before -executing the synchronize_rcu(), and the "newggp" value is also the -"ggp" value, but taken after the synchronize_rcu() command returns. - - -The output of "cat rcu/rcugp" looks as follows: - -na=1337955 nl=40 wa=1337915 wl=44 da=1337871 dl=0 dr=1337871 di=1337871 -1=50989 e1=6138 i1=49722 ie1=82 g1=49640 a1=315203 ae1=265563 a2=49640 -z1=1401244 ze1=1351605 z2=49639 m1=5661253 me1=5611614 m2=49639 - -These are counters tracking internal preemptable-RCU events, however, -some of them may be useful for debugging algorithms using RCU. In -particular, the "nl", "wl", and "dl" values track the number of RCU -callbacks in various states. The fields are as follows: - -o "na" is the total number of RCU callbacks that have been enqueued - since boot. - -o "nl" is the number of RCU callbacks waiting for the previous - grace period to end so that they can start waiting on the next - grace period. - -o "wa" is the total number of RCU callbacks that have started waiting - for a grace period since boot. "na" should be roughly equal to - "nl" plus "wa". - -o "wl" is the number of RCU callbacks currently waiting for their - grace period to end. - -o "da" is the total number of RCU callbacks whose grace periods - have completed since boot. "wa" should be roughly equal to - "wl" plus "da". - -o "dr" is the total number of RCU callbacks that have been removed - from the list of callbacks ready to invoke. "dr" should be roughly - equal to "da". - -o "di" is the total number of RCU callbacks that have been invoked - since boot. "di" should be roughly equal to "da", though some - early versions of preemptable RCU had a bug so that only the - last CPU's count of invocations was displayed, rather than the - sum of all CPU's counts. - -o "1" is the number of calls to rcu_try_flip(). This should be - roughly equal to the sum of "e1", "i1", "a1", "z1", and "m1" - described below. In other words, the number of times that - the state machine is visited should be equal to the sum of the - number of times that each state is visited plus the number of - times that the state-machine lock acquisition failed. - -o "e1" is the number of times that rcu_try_flip() was unable to - acquire the fliplock. - -o "i1" is the number of calls to rcu_try_flip_idle(). - -o "ie1" is the number of times rcu_try_flip_idle() exited early - due to the calling CPU having no work for RCU. - -o "g1" is the number of times that rcu_try_flip_idle() decided - to start a new grace period. "i1" should be roughly equal to - "ie1" plus "g1". - -o "a1" is the number of calls to rcu_try_flip_waitack(). - -o "ae1" is the number of times that rcu_try_flip_waitack() found - that at least one CPU had not yet acknowledge the new grace period - (AKA "counter flip"). - -o "a2" is the number of time rcu_try_flip_waitack() found that - all CPUs had acknowledged. "a1" should be roughly equal to - "ae1" plus "a2". (This particular output was collected on - a 128-CPU machine, hence the smaller-than-usual fraction of - calls to rcu_try_flip_waitack() finding all CPUs having already - acknowledged.) - -o "z1" is the number of calls to rcu_try_flip_waitzero(). - -o "ze1" is the number of times that rcu_try_flip_waitzero() found - that not all of the old RCU read-side critical sections had - completed. - -o "z2" is the number of times that rcu_try_flip_waitzero() finds - the sum of the counters equal to zero, in other words, that - all of the old RCU read-side critical sections had completed. - The value of "z1" should be roughly equal to "ze1" plus - "z2". - -o "m1" is the number of calls to rcu_try_flip_waitmb(). - -o "me1" is the number of times that rcu_try_flip_waitmb() finds - that at least one CPU has not yet executed a memory barrier. - -o "m2" is the number of times that rcu_try_flip_waitmb() finds that - all CPUs have executed a memory barrier. +The rcutree implementation of RCU provides debugfs trace output that +summarizes counters and state. This information is useful for debugging +RCU itself, and can sometimes also help to debug abuses of RCU. +The following sections describe the debugfs files and formats. Hierarchical RCU debugfs Files and Formats @@ -210,9 +35,10 @@ rcu_bh: 6 c=-275 g=-275 pq=1 pqc=-275 qp=0 dt=859/1 dn=0 df=15 of=0 ri=0 ql=0 b=10 7 c=-275 g=-275 pq=1 pqc=-275 qp=0 dt=3761/1 dn=0 df=15 of=0 ri=0 ql=0 b=10 -The first section lists the rcu_data structures for rcu, the second for -rcu_bh. Each section has one line per CPU, or eight for this 8-CPU system. -The fields are as follows: +The first section lists the rcu_data structures for rcu_sched, the second +for rcu_bh. Note that CONFIG_TREE_PREEMPT_RCU kernels will have an +additional section for rcu_preempt. Each section has one line per CPU, +or eight for this 8-CPU system. The fields are as follows: o The number at the beginning of each line is the CPU number. CPUs numbers followed by an exclamation mark are offline, @@ -223,9 +49,9 @@ o The number at the beginning of each line is the CPU number. o "c" is the count of grace periods that this CPU believes have completed. CPUs in dynticks idle mode may lag quite a ways - behind, for example, CPU 4 under "rcu" above, which has slept - through the past 25 RCU grace periods. It is not unusual to - see CPUs lagging by thousands of grace periods. + behind, for example, CPU 4 under "rcu_sched" above, which has + slept through the past 25 RCU grace periods. It is not unusual + to see CPUs lagging by thousands of grace periods. o "g" is the count of grace periods that this CPU believes have started. Again, CPUs in dynticks idle mode may lag behind. @@ -308,8 +134,10 @@ The output of "cat rcu/rcugp" looks as follows: rcu_sched: completed=33062 gpnum=33063 rcu_bh: completed=464 gpnum=464 -Again, this output is for both "rcu" and "rcu_bh". The fields are -taken from the rcu_state structure, and are as follows: +Again, this output is for both "rcu_sched" and "rcu_bh". Note that +kernels built with CONFIG_TREE_PREEMPT_RCU will have an additional +"rcu_preempt" line. The fields are taken from the rcu_state structure, +and are as follows: o "completed" is the number of grace periods that have completed. It is comparable to the "c" field from rcu/rcudata in that a @@ -324,23 +152,24 @@ o "gpnum" is the number of grace periods that have started. It is If these two fields are equal (as they are for "rcu_bh" above), then there is no grace period in progress, in other words, RCU is idle. On the other hand, if the two fields differ (as they - do for "rcu" above), then an RCU grace period is in progress. + do for "rcu_sched" above), then an RCU grace period is in progress. The output of "cat rcu/rcuhier" looks as follows, with very long lines: -c=6902 g=6903 s=2 jfq=3 j=72c7 nfqs=13142/nfqsng=0(13142) fqlh=6 +c=6902 g=6903 s=2 jfq=3 j=72c7 nfqs=13142/nfqsng=0(13142) fqlh=6 oqlen=0 1/1 0:127 ^0 3/3 0:35 ^0 0/0 36:71 ^1 0/0 72:107 ^2 0/0 108:127 ^3 3/3f 0:5 ^0 2/3 6:11 ^1 0/0 12:17 ^2 0/0 18:23 ^3 0/0 24:29 ^4 0/0 30:35 ^5 0/0 36:41 ^0 0/0 42:47 ^1 0/0 48:53 ^2 0/0 54:59 ^3 0/0 60:65 ^4 0/0 66:71 ^5 0/0 72:77 ^0 0/0 78:83 ^1 0/0 84:89 ^2 0/0 90:95 ^3 0/0 96:101 ^4 0/0 102:107 ^5 0/0 108:113 ^0 0/0 114:119 ^1 0/0 120:125 ^2 0/0 126:127 ^3 rcu_bh: -c=-226 g=-226 s=1 jfq=-5701 j=72c7 nfqs=88/nfqsng=0(88) fqlh=0 +c=-226 g=-226 s=1 jfq=-5701 j=72c7 nfqs=88/nfqsng=0(88) fqlh=0 oqlen=0 0/1 0:127 ^0 0/3 0:35 ^0 0/0 36:71 ^1 0/0 72:107 ^2 0/0 108:127 ^3 0/3f 0:5 ^0 0/3 6:11 ^1 0/0 12:17 ^2 0/0 18:23 ^3 0/0 24:29 ^4 0/0 30:35 ^5 0/0 36:41 ^0 0/0 42:47 ^1 0/0 48:53 ^2 0/0 54:59 ^3 0/0 60:65 ^4 0/0 66:71 ^5 0/0 72:77 ^0 0/0 78:83 ^1 0/0 84:89 ^2 0/0 90:95 ^3 0/0 96:101 ^4 0/0 102:107 ^5 0/0 108:113 ^0 0/0 114:119 ^1 0/0 120:125 ^2 0/0 126:127 ^3 -This is once again split into "rcu" and "rcu_bh" portions. The fields are -as follows: +This is once again split into "rcu_sched" and "rcu_bh" portions, +and CONFIG_TREE_PREEMPT_RCU kernels will again have an additional +"rcu_preempt" section. The fields are as follows: o "c" is exactly the same as "completed" under rcu/rcugp. @@ -372,6 +201,11 @@ o "fqlh" is the number of calls to force_quiescent_state() that exited immediately (without even being counted in nfqs above) due to contention on ->fqslock. +o "oqlen" is the number of callbacks on the "orphan" callback + list. RCU callbacks are placed on this list by CPUs going + offline, and are "adopted" either by the CPU helping the outgoing + CPU or by the next rcu_barrier*() call, whichever comes first. + o Each element of the form "1/1 0:127 ^0" represents one struct rcu_node. Each line represents one level of the hierarchy, from root to leaves. It is best to think of the rcu_data structures @@ -389,10 +223,10 @@ o Each element of the form "1/1 0:127 ^0" represents one struct The value of qsmaskinit is assigned to that of qsmask at the beginning of each grace period. - For example, for "rcu", the qsmask of the first entry - of the lowest level is 0x14, meaning that we are still - waiting for CPUs 2 and 4 to check in for the current - grace period. + For example, for "rcu_sched", the qsmask of the first + entry of the lowest level is 0x14, meaning that we + are still waiting for CPUs 2 and 4 to check in for the + current grace period. o The numbers separated by the ":" are the range of CPUs served by this struct rcu_node. This can be helpful @@ -431,8 +265,9 @@ rcu_bh: 6 np=120834 qsp=9902 cbr=0 cng=0 gpc=6 gps=3 nf=2 nn=110921 7 np=144888 qsp=26336 cbr=0 cng=0 gpc=8 gps=2 nf=0 nn=118542 -As always, this is once again split into "rcu" and "rcu_bh" portions. -The fields are as follows: +As always, this is once again split into "rcu_sched" and "rcu_bh" +portions, with CONFIG_TREE_PREEMPT_RCU kernels having an additional +"rcu_preempt" section. The fields are as follows: o "np" is the number of times that __rcu_pending() has been invoked for the corresponding flavor of RCU. |