CONFIG_RCU_TRACE debugfs Files and Formats


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

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_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.  CPUs in dynticks idle mode may lag quite a ways
	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.
	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_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 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 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_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 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	"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
	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 "T" preceding the ">"
		indicates that at least one task blocked in an RCU
		read-side critical section blocks the current grace
		period, while a "." preceding the ">" indicates otherwise.
		The character following the ">" indicates similarly for
		the next grace period.  A "T" should appear in this
		field only for rcu-preempt.

	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_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	"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().
