rcu: Update RCU's trace.txt documentation for new format

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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 five debugfs files under the
top-level directory RCU: rcu/rcudata (which displays fields in struct
rcu_data), rcu/rcudata.csv (which is a .csv spreadsheet version of
rcu/rcudata), rcu/rcugp (which displays grace-period counters),
rcu/rcuhier (which displays the struct rcu_node hierarchy), and
rcu/rcu_pending (which displays counts of the reasons that the
rcu_pending() function decided that there was core RCU work to do).

The output of "cat rcu/rcudata" looks as follows:

rcu_sched:
  0!c=423090 g=423091 pq=1 pqc=423090 qp=1 dt=86475/1/0 df=16319 of=163 ri=1519 ql=0 b=10 ci=1460693 co=1648 ca=6448
  1!c=423329 g=423330 pq=1 pqc=423329 qp=1 dt=90875/1/0 df=16231 of=157 ri=1249 ql=0 b=10 ci=1459002 co=1614 ca=3310
  2!c=423370 g=423371 pq=1 pqc=423370 qp=1 dt=69661/1/0 df=16125 of=163 ri=1469 ql=0 b=10 ci=1610701 co=2015 ca=2378
  3!c=422967 g=422968 pq=1 pqc=422967 qp=1 dt=70349/1/0 df=12528 of=163 ri=1450 ql=0 b=10 ci=1427543 co=1430 ca=897
  4!c=423196 g=423197 pq=1 pqc=423196 qp=0 dt=38935/1/0 df=10959 of=177 ri=1657 ql=0 b=10 ci=1562249 co=1896 ca=533
  5!c=422950 g=422951 pq=1 pqc=422950 qp=0 dt=25127/1/0 df=5895 of=167 ri=1549 ql=0 b=10 ci=1777260 co=2137 ca=274
  6!c=423396 g=423397 pq=1 pqc=423396 qp=1 dt=22639/1/0 df=4590 of=149 ri=1572 ql=0 b=10 ci=1471186 co=1530 ca=243
  7 c=460203 g=460203 pq=1 pqc=460202 qp=0 dt=937087/1/0 df=3298 of=149 ri=1584 ql=6 b=10 ci=4026154 co=1948 ca=135
rcu_bh:
  0!c=18446744073709551494 g=18446744073709551494 pq=0 pqc=18446744073709551493 qp=1 dt=86475/1/0 df=11 of=0 ri=0 ql=0 b=10 ci=112 co=0 ca=0
  1!c=18446744073709551496 g=18446744073709551496 pq=1 pqc=18446744073709551495 qp=0 dt=90875/1/0 df=15 of=0 ri=0 ql=0 b=10 ci=143 co=0 ca=0
  2!c=18446744073709551496 g=18446744073709551496 pq=1 pqc=18446744073709551495 qp=0 dt=69661/1/0 df=21 of=0 ri=1 ql=0 b=10 ci=88 co=0 ca=0
  3!c=18446744073709551494 g=18446744073709551494 pq=1 pqc=18446744073709551493 qp=0 dt=70349/1/0 df=13 of=0 ri=0 ql=0 b=10 ci=100 co=0 ca=0
  4!c=18446744073709551494 g=18446744073709551494 pq=0 pqc=18446744073709551493 qp=1 dt=38935/1/0 df=17 of=0 ri=0 ql=0 b=10 ci=36 co=0 ca=0
  5!c=18446744073709551494 g=18446744073709551494 pq=0 pqc=18446744073709551493 qp=1 dt=25127/1/0 df=7 of=0 ri=0 ql=0 b=10 ci=32 co=0 ca=0
  6!c=18446744073709551496 g=18446744073709551496 pq=1 pqc=18446744073709551495 qp=0 dt=22639/1/0 df=9 of=0 ri=0 ql=0 b=10 ci=44 co=0 ca=0
  7 c=182 g=182 pq=1 pqc=181 qp=0 dt=937087/1/0 df=14 of=0 ri=1 ql=0 b=10 ci=627 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	"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().


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 12 bits of the jiffies counter in hexadecimal.

o	"bt" is the low-order 12 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.