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diff --git a/Documentation/RCU/trace.txt b/Documentation/RCU/trace.txt new file mode 100644 index 00000000..8173cec4 --- /dev/null +++ b/Documentation/RCU/trace.txt @@ -0,0 +1,617 @@ +CONFIG_RCU_TRACE debugfs Files and Formats + + +The rcutree and rcutiny 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. +The following sections describe the debugfs files and formats, first +for rcutree and next for rcutiny. + + +CONFIG_TREE_RCU and CONFIG_TREE_PREEMPT_RCU debugfs Files and Formats + +These implementations of RCU provides several debugfs files under the +top-level directory "rcu": + +rcu/rcudata: + Displays fields in struct rcu_data. +rcu/rcudata.csv: + Comma-separated values spreadsheet version of rcudata. +rcu/rcugp: + Displays grace-period counters. +rcu/rcuhier: + Displays the struct rcu_node hierarchy. +rcu/rcu_pending: + Displays counts of the reasons rcu_pending() decided that RCU had + work to do. +rcu/rcutorture: + Displays rcutorture test progress. +rcu/rcuboost: + Displays RCU boosting statistics. Only present if + CONFIG_RCU_BOOST=y. + +The output of "cat rcu/rcudata" looks as follows: + +rcu_sched: + 0 c=20972 g=20973 pq=1 pqc=20972 qp=0 dt=545/1/0 df=50 of=0 ri=0 ql=163 qs=NRW. kt=0/W/0 ktl=ebc3 b=10 ci=153737 co=0 ca=0 + 1 c=20972 g=20973 pq=1 pqc=20972 qp=0 dt=967/1/0 df=58 of=0 ri=0 ql=634 qs=NRW. kt=0/W/1 ktl=58c b=10 ci=191037 co=0 ca=0 + 2 c=20972 g=20973 pq=1 pqc=20972 qp=0 dt=1081/1/0 df=175 of=0 ri=0 ql=74 qs=N.W. kt=0/W/2 ktl=da94 b=10 ci=75991 co=0 ca=0 + 3 c=20942 g=20943 pq=1 pqc=20942 qp=1 dt=1846/0/0 df=404 of=0 ri=0 ql=0 qs=.... kt=0/W/3 ktl=d1cd b=10 ci=72261 co=0 ca=0 + 4 c=20972 g=20973 pq=1 pqc=20972 qp=0 dt=369/1/0 df=83 of=0 ri=0 ql=48 qs=N.W. kt=0/W/4 ktl=e0e7 b=10 ci=128365 co=0 ca=0 + 5 c=20972 g=20973 pq=1 pqc=20972 qp=0 dt=381/1/0 df=64 of=0 ri=0 ql=169 qs=NRW. kt=0/W/5 ktl=fb2f b=10 ci=164360 co=0 ca=0 + 6 c=20972 g=20973 pq=1 pqc=20972 qp=0 dt=1037/1/0 df=183 of=0 ri=0 ql=62 qs=N.W. kt=0/W/6 ktl=d2ad b=10 ci=65663 co=0 ca=0 + 7 c=20897 g=20897 pq=1 pqc=20896 qp=0 dt=1572/0/0 df=382 of=0 ri=0 ql=0 qs=.... kt=0/W/7 ktl=cf15 b=10 ci=75006 co=0 ca=0 +rcu_bh: + 0 c=1480 g=1480 pq=1 pqc=1479 qp=0 dt=545/1/0 df=6 of=0 ri=1 ql=0 qs=.... kt=0/W/0 ktl=ebc3 b=10 ci=0 co=0 ca=0 + 1 c=1480 g=1480 pq=1 pqc=1479 qp=0 dt=967/1/0 df=3 of=0 ri=1 ql=0 qs=.... kt=0/W/1 ktl=58c b=10 ci=151 co=0 ca=0 + 2 c=1480 g=1480 pq=1 pqc=1479 qp=0 dt=1081/1/0 df=6 of=0 ri=1 ql=0 qs=.... kt=0/W/2 ktl=da94 b=10 ci=0 co=0 ca=0 + 3 c=1480 g=1480 pq=1 pqc=1479 qp=0 dt=1846/0/0 df=8 of=0 ri=1 ql=0 qs=.... kt=0/W/3 ktl=d1cd b=10 ci=0 co=0 ca=0 + 4 c=1480 g=1480 pq=1 pqc=1479 qp=0 dt=369/1/0 df=6 of=0 ri=1 ql=0 qs=.... kt=0/W/4 ktl=e0e7 b=10 ci=0 co=0 ca=0 + 5 c=1480 g=1480 pq=1 pqc=1479 qp=0 dt=381/1/0 df=4 of=0 ri=1 ql=0 qs=.... kt=0/W/5 ktl=fb2f b=10 ci=0 co=0 ca=0 + 6 c=1480 g=1480 pq=1 pqc=1479 qp=0 dt=1037/1/0 df=6 of=0 ri=1 ql=0 qs=.... kt=0/W/6 ktl=d2ad b=10 ci=0 co=0 ca=0 + 7 c=1474 g=1474 pq=1 pqc=1473 qp=0 dt=1572/0/0 df=8 of=0 ri=1 ql=0 qs=.... kt=0/W/7 ktl=cf15 b=10 ci=0 co=0 ca=0 + +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, + but have been online at least once since boot. There will be + no output for CPUs that have never been online, which can be + a good thing in the surprisingly common case where NR_CPUS is + substantially larger than the number of actual CPUs. + +o "c" is the count of grace periods that this CPU believes have + completed. Offlined CPUs and CPUs in dynticks idle mode may + lag quite a ways behind, for example, CPU 6 under "rcu_sched" + above, which has been offline through not quite 40,000 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, offlined CPUs and CPUs in dynticks idle mode + may lag behind. If the "c" and "g" values are equal, this CPU + has already reported a quiescent state for the last RCU grace + period that it is aware of, otherwise, the CPU believes that it + owes RCU a quiescent state. + +o "pq" indicates that this CPU has passed through a quiescent state + for the current grace period. It is possible for "pq" to be + "1" and "c" different than "g", which indicates that although + the CPU has passed through a quiescent state, either (1) this + CPU has not yet reported that fact, (2) some other CPU has not + yet reported for this grace period, or (3) both. + +o "pqc" indicates which grace period the last-observed quiescent + state for this CPU corresponds to. This is important for handling + the race between CPU 0 reporting an extended dynticks-idle + quiescent state for CPU 1 and CPU 1 suddenly waking up and + reporting its own quiescent state. If CPU 1 was the last CPU + for the current grace period, then the CPU that loses this race + will attempt to incorrectly mark CPU 1 as having checked in for + the next grace period! + +o "qp" indicates that RCU still expects a quiescent state from + this CPU. Offlined CPUs and CPUs in dyntick idle mode might + well have qp=1, which is OK: RCU is still ignoring them. + +o "dt" is the current value of the dyntick counter that is incremented + when entering or leaving dynticks idle state, either by the + scheduler or by irq. This number is even if the CPU is in + dyntick idle mode and odd otherwise. The number after the first + "/" is the interrupt nesting depth when in dyntick-idle state, + or one greater than the interrupt-nesting depth otherwise. + The number after the second "/" is the NMI nesting depth. + + This field is displayed only for CONFIG_NO_HZ kernels. + +o "df" is the number of times that some other CPU has forced a + quiescent state on behalf of this CPU due to this CPU being in + dynticks-idle state. + + This field is displayed only for CONFIG_NO_HZ kernels. + +o "of" is the number of times that some other CPU has forced a + quiescent state on behalf of this CPU due to this CPU being + offline. In a perfect world, this might never happen, but it + turns out that offlining and onlining a CPU can take several grace + periods, and so there is likely to be an extended period of time + when RCU believes that the CPU is online when it really is not. + Please note that erring in the other direction (RCU believing a + CPU is offline when it is really alive and kicking) is a fatal + error, so it makes sense to err conservatively. + +o "ri" is the number of times that RCU has seen fit to send a + reschedule IPI to this CPU in order to get it to report a + quiescent state. + +o "ql" is the number of RCU callbacks currently residing on + this CPU. This is the total number of callbacks, regardless + of what state they are in (new, waiting for grace period to + start, waiting for grace period to end, ready to invoke). + +o "qs" gives an indication of the state of the callback queue + with four characters: + + "N" Indicates that there are callbacks queued that are not + ready to be handled by the next grace period, and thus + will be handled by the grace period following the next + one. + + "R" Indicates that there are callbacks queued that are + ready to be handled by the next grace period. + + "W" Indicates that there are callbacks queued that are + waiting on the current grace period. + + "D" Indicates that there are callbacks queued that have + already been handled by a prior grace period, and are + thus waiting to be invoked. Note that callbacks in + the process of being invoked are not counted here. + Callbacks in the process of being invoked are those + that have been removed from the rcu_data structures + queues by rcu_do_batch(), but which have not yet been + invoked. + + If there are no callbacks in a given one of the above states, + the corresponding character is replaced by ".". + +o "kt" is the per-CPU kernel-thread state. The digit preceding + the first slash is zero if there is no work pending and 1 + otherwise. The character between the first pair of slashes is + as follows: + + "S" The kernel thread is stopped, in other words, all + CPUs corresponding to this rcu_node structure are + offline. + + "R" The kernel thread is running. + + "W" The kernel thread is waiting because there is no work + for it to do. + + "O" The kernel thread is waiting because it has been + forced off of its designated CPU or because its + ->cpus_allowed mask permits it to run on other than + its designated CPU. + + "Y" The kernel thread is yielding to avoid hogging CPU. + + "?" Unknown value, indicates a bug. + + The number after the final slash is the CPU that the kthread + is actually running on. + +o "ktl" is the low-order 16 bits (in hexadecimal) of the count of + the number of times that this CPU's per-CPU kthread has gone + through its loop servicing invoke_rcu_cpu_kthread() requests. + +o "b" is the batch limit for this CPU. If more than this number + of RCU callbacks is ready to invoke, then the remainder will + be deferred. + +o "ci" is the number of RCU callbacks that have been invoked for + this CPU. Note that ci+ql is the number of callbacks that have + been registered in absence of CPU-hotplug activity. + +o "co" is the number of RCU callbacks that have been orphaned due to + this CPU going offline. These orphaned callbacks have been moved + to an arbitrarily chosen online CPU. + +o "ca" is the number of RCU callbacks that have been adopted due to + other CPUs going offline. Note that ci+co-ca+ql is the number of + RCU callbacks registered on this CPU. + +There is also an rcu/rcudata.csv file with the same information in +comma-separated-variable spreadsheet format. + + +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_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 + CPU whose "c" field matches the value of "completed" is aware + that the corresponding RCU grace period has completed. + +o "gpnum" is the number of grace periods that have started. It is + comparable to the "g" field from rcu/rcudata in that a CPU + whose "g" field matches the value of "gpnum" is aware that the + corresponding RCU grace period has started. + + 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_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 +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 +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_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. + +o "g" is exactly the same as "gpnum" under rcu/rcugp. + +o "s" is the "signaled" state that drives force_quiescent_state()'s + state machine. + +o "jfq" is the number of jiffies remaining for this grace period + before force_quiescent_state() is invoked to help push things + along. Note that CPUs in dyntick-idle mode throughout the grace + period will not report on their own, but rather must be check by + some other CPU via force_quiescent_state(). + +o "j" is the low-order four hex digits of the jiffies counter. + Yes, Paul did run into a number of problems that turned out to + be due to the jiffies counter no longer counting. Why do you ask? + +o "nfqs" is the number of calls to force_quiescent_state() since + boot. + +o "nfqsng" is the number of useless calls to force_quiescent_state(), + where there wasn't actually a grace period active. This can + happen due to races. The number in parentheses is the difference + between "nfqs" and "nfqsng", or the number of times that + force_quiescent_state() actually did some real work. + +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 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 + as forming yet another level after the leaves. Note that there + might be either one, two, or three levels of rcu_node structures, + depending on the relationship between CONFIG_RCU_FANOUT and + CONFIG_NR_CPUS. + + o The numbers separated by the "/" are the qsmask followed + by the qsmaskinit. The qsmask will have one bit + set for each entity in the next lower level that + has not yet checked in for the current grace period. + The qsmaskinit will have one bit for each entity that is + currently expected to check in during each grace period. + The value of qsmaskinit is assigned to that of qsmask + at the beginning of each 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 characters separated by the ">" indicate the state + of the blocked-tasks lists. A "G" preceding the ">" + indicates that at least one task blocked in an RCU + read-side critical section blocks the current grace + period, while a "E" preceding the ">" indicates that + at least one task blocked in an RCU read-side critical + section blocks the current expedited grace period. + A "T" character following the ">" indicates that at + least one task is blocked within an RCU read-side + critical section, regardless of whether any current + grace period (expedited or normal) is inconvenienced. + A "." character appears if the corresponding condition + does not hold, so that "..>." indicates that no tasks + are blocked. In contrast, "GE>T" indicates maximal + inconvenience from blocked tasks. + + o The numbers separated by the ":" are the range of CPUs + served by this struct rcu_node. This can be helpful + in working out how the hierarchy is wired together. + + For example, the first entry at the lowest level shows + "0:5", indicating that it covers CPUs 0 through 5. + + o The number after the "^" indicates the bit in the + next higher level rcu_node structure that this + rcu_node structure corresponds to. + + For example, the first entry at the lowest level shows + "^0", indicating that it corresponds to bit zero in + the first entry at the middle level. + + +The output of "cat rcu/rcu_pending" looks as follows: + +rcu_sched: + 0 np=255892 qsp=53936 rpq=85 cbr=0 cng=14417 gpc=10033 gps=24320 nf=6445 nn=146741 + 1 np=261224 qsp=54638 rpq=33 cbr=0 cng=25723 gpc=16310 gps=2849 nf=5912 nn=155792 + 2 np=237496 qsp=49664 rpq=23 cbr=0 cng=2762 gpc=45478 gps=1762 nf=1201 nn=136629 + 3 np=236249 qsp=48766 rpq=98 cbr=0 cng=286 gpc=48049 gps=1218 nf=207 nn=137723 + 4 np=221310 qsp=46850 rpq=7 cbr=0 cng=26 gpc=43161 gps=4634 nf=3529 nn=123110 + 5 np=237332 qsp=48449 rpq=9 cbr=0 cng=54 gpc=47920 gps=3252 nf=201 nn=137456 + 6 np=219995 qsp=46718 rpq=12 cbr=0 cng=50 gpc=42098 gps=6093 nf=4202 nn=120834 + 7 np=249893 qsp=49390 rpq=42 cbr=0 cng=72 gpc=38400 gps=17102 nf=41 nn=144888 +rcu_bh: + 0 np=146741 qsp=1419 rpq=6 cbr=0 cng=6 gpc=0 gps=0 nf=2 nn=145314 + 1 np=155792 qsp=12597 rpq=3 cbr=0 cng=0 gpc=4 gps=8 nf=3 nn=143180 + 2 np=136629 qsp=18680 rpq=1 cbr=0 cng=0 gpc=7 gps=6 nf=0 nn=117936 + 3 np=137723 qsp=2843 rpq=0 cbr=0 cng=0 gpc=10 gps=7 nf=0 nn=134863 + 4 np=123110 qsp=12433 rpq=0 cbr=0 cng=0 gpc=4 gps=2 nf=0 nn=110671 + 5 np=137456 qsp=4210 rpq=1 cbr=0 cng=0 gpc=6 gps=5 nf=0 nn=133235 + 6 np=120834 qsp=9902 rpq=2 cbr=0 cng=0 gpc=6 gps=3 nf=2 nn=110921 + 7 np=144888 qsp=26336 rpq=0 cbr=0 cng=0 gpc=8 gps=2 nf=0 nn=118542 + +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. + +o "qsp" is the number of times that the RCU was waiting for a + quiescent state from this CPU. + +o "rpq" is the number of times that the CPU had passed through + a quiescent state, but not yet reported it to RCU. + +o "cbr" is the number of times that this CPU had RCU callbacks + that had passed through a grace period, and were thus ready + to be invoked. + +o "cng" is the number of times that this CPU needed another + grace period while RCU was idle. + +o "gpc" is the number of times that an old grace period had + completed, but this CPU was not yet aware of it. + +o "gps" is the number of times that a new grace period had started, + but this CPU was not yet aware of it. + +o "nf" is the number of times that this CPU suspected that the + current grace period had run for too long, and thus needed to + be forced. + + Please note that "forcing" consists of sending resched IPIs + to holdout CPUs. If that CPU really still is in an old RCU + read-side critical section, then we really do have to wait for it. + The assumption behing "forcing" is that the CPU is not still in + an old RCU read-side critical section, but has not yet responded + for some other reason. + +o "nn" is the number of times that this CPU needed nothing. Alert + readers will note that the rcu "nn" number for a given CPU very + closely matches the rcu_bh "np" number for that same CPU. This + is due to short-circuit evaluation in rcu_pending(). + + +The output of "cat rcu/rcutorture" looks as follows: + +rcutorture test sequence: 0 (test in progress) +rcutorture update version number: 615 + +The first line shows the number of rcutorture tests that have completed +since boot. If a test is currently running, the "(test in progress)" +string will appear as shown above. The second line shows the number of +update cycles that the current test has started, or zero if there is +no test in progress. + + +The output of "cat rcu/rcuboost" looks as follows: + +0:5 tasks=.... kt=W ntb=0 neb=0 nnb=0 j=2f95 bt=300f + balk: nt=0 egt=989 bt=0 nb=0 ny=0 nos=16 +6:7 tasks=.... kt=W ntb=0 neb=0 nnb=0 j=2f95 bt=300f + balk: nt=0 egt=225 bt=0 nb=0 ny=0 nos=6 + +This information is output only for rcu_preempt. Each two-line entry +corresponds to a leaf rcu_node strcuture. The fields are as follows: + +o "n:m" is the CPU-number range for the corresponding two-line + entry. In the sample output above, the first entry covers + CPUs zero through five and the second entry covers CPUs 6 + and 7. + +o "tasks=TNEB" gives the state of the various segments of the + rnp->blocked_tasks list: + + "T" This indicates that there are some tasks that blocked + while running on one of the corresponding CPUs while + in an RCU read-side critical section. + + "N" This indicates that some of the blocked tasks are preventing + the current normal (non-expedited) grace period from + completing. + + "E" This indicates that some of the blocked tasks are preventing + the current expedited grace period from completing. + + "B" This indicates that some of the blocked tasks are in + need of RCU priority boosting. + + Each character is replaced with "." if the corresponding + condition does not hold. + +o "kt" is the state of the RCU priority-boosting kernel + thread associated with the corresponding rcu_node structure. + The state can be one of the following: + + "S" The kernel thread is stopped, in other words, all + CPUs corresponding to this rcu_node structure are + offline. + + "R" The kernel thread is running. + + "W" The kernel thread is waiting because there is no work + for it to do. + + "Y" The kernel thread is yielding to avoid hogging CPU. + + "?" Unknown value, indicates a bug. + +o "ntb" is the number of tasks boosted. + +o "neb" is the number of tasks boosted in order to complete an + expedited grace period. + +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, + it is counted against the expedited total above. + +o "j" is the low-order 16 bits of the jiffies counter in + hexadecimal. + +o "bt" is the low-order 16 bits of the value that the jiffies + counter will have when we next start boosting, assuming that + the current grace period does not end beforehand. This is + also in hexadecimal. + +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. + +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. + +o "bt" counts the number of times we balked because boosting + had already been initiated for the current grace period. + +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! + +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. + + +CONFIG_TINY_RCU and CONFIG_TINY_PREEMPT_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, rcu_sched_ctrlblk and, for CONFIG_TINY_PREEMPT_RCU, +rcu_preempt_ctrlblk. + +The output of "cat rcu/rcudata" is as follows: + +rcu_preempt: qlen=24 gp=1097669 g197/p197/c197 tasks=... + ttb=. btg=no ntb=184 neb=0 nnb=183 j=01f7 bt=0274 + normal balk: nt=1097669 gt=0 bt=371 b=0 ny=25073378 nos=0 + exp balk: bt=0 nos=0 +rcu_sched: qlen: 0 +rcu_bh: qlen: 0 + +This is split into rcu_preempt, rcu_sched, and rcu_bh sections, with the +rcu_preempt section appearing only in CONFIG_TINY_PREEMPT_RCU builds. +The last three lines of the rcu_preempt section appear only in +CONFIG_RCU_BOOST kernel builds. The fields are as follows: + +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. + +o "gp" is the number of grace periods that have completed. + +o "g197/p197/c197" displays the grace-period state, with the + "g" number being the number of grace periods that have started + (mod 256), the "p" number being the number of grace periods + that the CPU has responded to (also mod 256), and the "c" + number being the number of grace periods that have completed + (once again mode 256). + + Why have both "gp" and "g"? Because the data flowing into + "gp" is only present in a CONFIG_RCU_TRACE kernel. + +o "tasks" is a set of bits. The first bit is "T" if there are + currently tasks that have recently blocked within an RCU + read-side critical section, the second bit is "N" if any of the + aforementioned tasks are blocking the current RCU grace period, + and the third bit is "E" if any of the aforementioned tasks are + blocking the current expedited grace period. Each bit is "." + if the corresponding condition does not hold. + +o "ttb" is a single bit. It is "B" if any of the blocked tasks + need to be priority boosted and "." otherwise. + +o "btg" indicates whether boosting has been carried out during + the current grace period, with "exp" indicating that boosting + is in progress for an expedited grace period, "no" indicating + that boosting has not yet started for a normal grace period, + "begun" indicating that boosting has bebug for a normal grace + period, and "done" indicating that boosting has completed for + a normal grace period. + +o "ntb" is the total number of tasks subjected to RCU priority boosting + periods since boot. + +o "neb" is the number of expedited grace periods that have had + to resort to RCU priority boosting since boot. + +o "nnb" is the number of normal grace periods that have had + to resort to RCU priority boosting since boot. + +o "j" is the low-order 16 bits of the jiffies counter in hexadecimal. + +o "bt" is the low-order 16 bits of the value that the jiffies counter + will have at the next time that boosting is scheduled to begin. + +o In the line beginning with "normal balk", the fields are as follows: + + o "nt" is the number of times that the system balked from + boosting because there were no blocked tasks to boost. + Note that the system will balk from boosting even if the + grace period is overdue when the currently running task + is looping within an RCU read-side critical section. + There is no point in boosting in this case, because + boosting a running task won't make it run any faster. + + o "gt" is the number of times that the system balked + from boosting because, although there were blocked tasks, + none of them were preventing the current grace period + from completing. + + o "bt" is the number of times that the system balked + from boosting because boosting was already in progress. + + o "b" is the number of times that the system balked from + boosting because boosting had already completed for + the grace period in question. + + o "ny" is the number of times that the system balked from + boosting because it was not yet time to start boosting + the grace period in question. + + o "nos" is the number of times that the system balked from + boosting for inexplicable ("not otherwise specified") + reasons. This can actually happen due to races involving + increments of the jiffies counter. + +o In the line beginning with "exp balk", the fields are as follows: + + o "bt" is the number of times that the system balked from + boosting because there were no blocked tasks to boost. + + o "nos" is the number of times that the system balked from + boosting for inexplicable ("not otherwise specified") + reasons. |