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. Hierarchical RCU debugfs Files and Formats This implementation of RCU provides three debugfs files under the top-level directory RCU: rcu/rcudata (which displays fields in struct rcu_data), rcu/rcugp (which displays grace-period counters), and rcu/rcuhier (which displays the struct rcu_node hierarchy). The output of "cat rcu/rcudata" looks as follows: rcu: 0 c=4011 g=4012 pq=1 pqc=4011 qp=0 rpfq=1 rp=3c2a dt=23301/73 dn=2 df=1882 of=0 ri=2126 ql=2 b=10 1 c=4011 g=4012 pq=1 pqc=4011 qp=0 rpfq=3 rp=39a6 dt=78073/1 dn=2 df=1402 of=0 ri=1875 ql=46 b=10 2 c=4010 g=4010 pq=1 pqc=4010 qp=0 rpfq=-5 rp=1d12 dt=16646/0 dn=2 df=3140 of=0 ri=2080 ql=0 b=10 3 c=4012 g=4013 pq=1 pqc=4012 qp=1 rpfq=3 rp=2b50 dt=21159/1 dn=2 df=2230 of=0 ri=1923 ql=72 b=10 4 c=4012 g=4013 pq=1 pqc=4012 qp=1 rpfq=3 rp=1644 dt=5783/1 dn=2 df=3348 of=0 ri=2805 ql=7 b=10 5 c=4012 g=4013 pq=0 pqc=4011 qp=1 rpfq=3 rp=1aac dt=5879/1 dn=2 df=3140 of=0 ri=2066 ql=10 b=10 6 c=4012 g=4013 pq=1 pqc=4012 qp=1 rpfq=3 rp=ed8 dt=5847/1 dn=2 df=3797 of=0 ri=1266 ql=10 b=10 7 c=4012 g=4013 pq=1 pqc=4012 qp=1 rpfq=3 rp=1fa2 dt=6199/1 dn=2 df=2795 of=0 ri=2162 ql=28 b=10 rcu_bh: 0 c=-268 g=-268 pq=1 pqc=-268 qp=0 rpfq=-145 rp=21d6 dt=23301/73 dn=2 df=0 of=0 ri=0 ql=0 b=10 1 c=-268 g=-268 pq=1 pqc=-268 qp=1 rpfq=-170 rp=20ce dt=78073/1 dn=2 df=26 of=0 ri=5 ql=0 b=10 2 c=-268 g=-268 pq=1 pqc=-268 qp=1 rpfq=-83 rp=fbd dt=16646/0 dn=2 df=28 of=0 ri=4 ql=0 b=10 3 c=-268 g=-268 pq=1 pqc=-268 qp=0 rpfq=-105 rp=178c dt=21159/1 dn=2 df=28 of=0 ri=2 ql=0 b=10 4 c=-268 g=-268 pq=1 pqc=-268 qp=1 rpfq=-30 rp=b54 dt=5783/1 dn=2 df=32 of=0 ri=0 ql=0 b=10 5 c=-268 g=-268 pq=1 pqc=-268 qp=1 rpfq=-29 rp=df5 dt=5879/1 dn=2 df=30 of=0 ri=3 ql=0 b=10 6 c=-268 g=-268 pq=1 pqc=-268 qp=1 rpfq=-28 rp=788 dt=5847/1 dn=2 df=32 of=0 ri=0 ql=0 b=10 7 c=-268 g=-268 pq=1 pqc=-268 qp=1 rpfq=-53 rp=1098 dt=6199/1 dn=2 df=30 of=0 ri=3 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: 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. 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. o "g" is the count of grace periods that this CPU believes have started. Again, 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. o "rpfq" is the number of rcu_pending() calls on this CPU required to induce this CPU to invoke force_quiescent_state(). o "rp" is low-order four hex digits of the count of how many times rcu_pending() has been invoked on this CPU. 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. The number after the "/" is the interrupt nesting depth when in dyntick-idle state, or one greater than the interrupt-nesting depth otherwise. This field is displayed only for CONFIG_NO_HZ kernels. o "dn" is the current value of the dyntick counter that is incremented when entering or leaving dynticks idle state via NMI. If both the "dt" and "dn" values are even, then this CPU is in dynticks idle mode and may be ignored by RCU. If either of these two counters is odd, then RCU must be alert to the possibility of an RCU read-side critical section running on this CPU. 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 neve 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 "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. The output of "cat rcu/rcugp" looks as follows: rcu: 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: 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" 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" and "rcu_bh" portions. 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 thoughout 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", 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 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.