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_sched:
0 c=17829 g=17829 pq=1 pqc=17829 qp=0 dt=10951/1 dn=0 df=1101 of=0 ri=36 ql=0 b=10
1 c=17829 g=17829 pq=1 pqc=17829 qp=0 dt=16117/1 dn=0 df=1015 of=0 ri=0 ql=0 b=10
2 c=17829 g=17829 pq=1 pqc=17829 qp=0 dt=1445/1 dn=0 df=1839 of=0 ri=0 ql=0 b=10
3 c=17829 g=17829 pq=1 pqc=17829 qp=0 dt=6681/1 dn=0 df=1545 of=0 ri=0 ql=0 b=10
4 c=17829 g=17829 pq=1 pqc=17829 qp=0 dt=1003/1 dn=0 df=1992 of=0 ri=0 ql=0 b=10
5 c=17829 g=17830 pq=1 pqc=17829 qp=1 dt=3887/1 dn=0 df=3331 of=0 ri=4 ql=2 b=10
6 c=17829 g=17829 pq=1 pqc=17829 qp=0 dt=859/1 dn=0 df=3224 of=0 ri=0 ql=0 b=10
7 c=17829 g=17830 pq=0 pqc=17829 qp=1 dt=3761/1 dn=0 df=1818 of=0 ri=0 ql=2 b=10
rcu_bh:
0 c=-275 g=-275 pq=1 pqc=-275 qp=0 dt=10951/1 dn=0 df=0 of=0 ri=0 ql=0 b=10
1 c=-275 g=-275 pq=1 pqc=-275 qp=0 dt=16117/1 dn=0 df=13 of=0 ri=0 ql=0 b=10
2 c=-275 g=-275 pq=1 pqc=-275 qp=0 dt=1445/1 dn=0 df=15 of=0 ri=0 ql=0 b=10
3 c=-275 g=-275 pq=1 pqc=-275 qp=0 dt=6681/1 dn=0 df=9 of=0 ri=0 ql=0 b=10
4 c=-275 g=-275 pq=1 pqc=-275 qp=0 dt=1003/1 dn=0 df=15 of=0 ri=0 ql=0 b=10
5 c=-275 g=-275 pq=1 pqc=-275 qp=0 dt=3887/1 dn=0 df=15 of=0 ri=0 ql=0 b=10
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:
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 “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.
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” 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.
The output of “cat rcu/rcu_pending” looks as follows:
rcu_sched:
0 np=255892 qsp=53936 cbr=0 cng=14417 gpc=10033 gps=24320 nf=6445 nn=146741
1 np=261224 qsp=54638 cbr=0 cng=25723 gpc=16310 gps=2849 nf=5912 nn=155792
2 np=237496 qsp=49664 cbr=0 cng=2762 gpc=45478 gps=1762 nf=1201 nn=136629
3 np=236249 qsp=48766 cbr=0 cng=286 gpc=48049 gps=1218 nf=207 nn=137723
4 np=221310 qsp=46850 cbr=0 cng=26 gpc=43161 gps=4634 nf=3529 nn=123110
5 np=237332 qsp=48449 cbr=0 cng=54 gpc=47920 gps=3252 nf=201 nn=137456
6 np=219995 qsp=46718 cbr=0 cng=50 gpc=42098 gps=6093 nf=4202 nn=120834
7 np=249893 qsp=49390 cbr=0 cng=72 gpc=38400 gps=17102 nf=41 nn=144888
rcu_bh:
0 np=146741 qsp=1419 cbr=0 cng=6 gpc=0 gps=0 nf=2 nn=145314
1 np=155792 qsp=12597 cbr=0 cng=0 gpc=4 gps=8 nf=3 nn=143180
2 np=136629 qsp=18680 cbr=0 cng=0 gpc=7 gps=6 nf=0 nn=117936
3 np=137723 qsp=2843 cbr=0 cng=0 gpc=10 gps=7 nf=0 nn=134863
4 np=123110 qsp=12433 cbr=0 cng=0 gpc=4 gps=2 nf=0 nn=110671
5 np=137456 qsp=4210 cbr=0 cng=0 gpc=6 gps=5 nf=0 nn=133235
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:
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 “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().
