path: root/Documentation/RCU/trace.txt

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.