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 provide several debugfs directories under the
  top-level directory "rcu":
  
  rcu/rcu_bh
  rcu/rcu_preempt
  rcu/rcu_sched
  
  Each directory contains files for the corresponding flavor of RCU.
  Note that rcu/rcu_preempt is only present for CONFIG_TREE_PREEMPT_RCU.
  For CONFIG_TREE_RCU, the RCU flavor maps onto the RCU-sched flavor,
  so that activity for both appears in rcu/rcu_sched.
  
  In addition, the following file appears in the top-level directory:
  rcu/rcutorture.  This file displays rcutorture test progress.  The output
  of "cat rcu/rcutorture" looks as follows:
  
  rcutorture test sequence: 0 (test in progress)
  rcutorture update version number: 615
  
  The first line shows the number of rcutorture tests that have completed
  since boot.  If a test is currently running, the "(test in progress)"
  string will appear as shown above.  The second line shows the number of
  update cycles that the current test has started, or zero if there is
  no test in progress.
  
  
  Within each flavor directory (rcu/rcu_bh, rcu/rcu_sched, and possibly
  also rcu/rcu_preempt) the following files will be present:
  
  rcudata:
  	Displays fields in struct rcu_data.
  rcuexp:
  	Displays statistics for expedited grace periods.
  rcugp:
  	Displays grace-period counters.
  rcuhier:
  	Displays the struct rcu_node hierarchy.
  rcu_pending:
  	Displays counts of the reasons rcu_pending() decided that RCU had
  	work to do.
  rcuboost:
  	Displays RCU boosting statistics.  Only present if
  	CONFIG_RCU_BOOST=y.
  
  The output of "cat rcu/rcu_preempt/rcudata" looks as follows:
  
    0!c=30455 g=30456 pq=1 qp=1 dt=126535/140000000000000/0 df=2002 of=4 ql=0/0 qs=N... b=10 ci=74572 nci=0 co=1131 ca=716
    1!c=30719 g=30720 pq=1 qp=0 dt=132007/140000000000000/0 df=1874 of=10 ql=0/0 qs=N... b=10 ci=123209 nci=0 co=685 ca=982
    2!c=30150 g=30151 pq=1 qp=1 dt=138537/140000000000000/0 df=1707 of=8 ql=0/0 qs=N... b=10 ci=80132 nci=0 co=1328 ca=1458
    3 c=31249 g=31250 pq=1 qp=0 dt=107255/140000000000000/0 df=1749 of=6 ql=0/450 qs=NRW. b=10 ci=151700 nci=0 co=509 ca=622
    4!c=29502 g=29503 pq=1 qp=1 dt=83647/140000000000000/0 df=965 of=5 ql=0/0 qs=N... b=10 ci=65643 nci=0 co=1373 ca=1521
    5 c=31201 g=31202 pq=1 qp=1 dt=70422/0/0 df=535 of=7 ql=0/0 qs=.... b=10 ci=58500 nci=0 co=764 ca=698
    6!c=30253 g=30254 pq=1 qp=1 dt=95363/140000000000000/0 df=780 of=5 ql=0/0 qs=N... b=10 ci=100607 nci=0 co=1414 ca=1353
    7 c=31178 g=31178 pq=1 qp=0 dt=91536/0/0 df=547 of=4 ql=0/0 qs=.... b=10 ci=109819 nci=0 co=1115 ca=969
  
  This file 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 4 under "rcu_sched" above,
  	which has been offline through 16 RCU grace periods.  It is not
  	unusual to see offline CPUs lagging by thousands of grace periods.
  	Note that although the grace-period number is an unsigned long,
  	it is printed out as a signed long to allow more human-friendly
  	representation near boot time.
  
  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	"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 idle, either due to a context switch or
  	due to an interrupt.  This number is even if the CPU is in idle
  	from RCU's viewpoint and odd otherwise.  The number after the
  	first "/" is the interrupt nesting depth when in idle state,
  	or a large number added to the interrupt-nesting depth when
  	running a non-idle task.  Some architectures do not accurately
  	count interrupt nesting when running in non-idle kernel context,
  	which can result in interesting anomalies such as negative
  	interrupt-nesting levels.  The number after the second "/"
  	is the NMI nesting depth.
  
  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
  	idle state.
  
  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	"ql" is the number of RCU callbacks currently residing on
  	this CPU.  The first number is the number of "lazy" callbacks
  	that are known to RCU to only be freeing memory, and the number
  	after the "/" is the total number of callbacks, lazy or not.
  	These counters count callbacks regardless of what phase of
  	grace-period processing that they are in (new, waiting for
  	grace period to start, waiting for grace period to end, ready
  	to invoke).
  
  o	"qs" gives an indication of the state of the callback queue
  	with four characters:
  
  	"N"	Indicates that there are callbacks queued that are not
  		ready to be handled by the next grace period, and thus
  		will be handled by the grace period following the next
  		one.
  
  	"R"	Indicates that there are callbacks queued that are
  		ready to be handled by the next grace period.
  
  	"W"	Indicates that there are callbacks queued that are
  		waiting on the current grace period.
  
  	"D"	Indicates that there are callbacks queued that have
  		already been handled by a prior grace period, and are
  		thus waiting to be invoked.  Note that callbacks in
  		the process of being invoked are not counted here.
  		Callbacks in the process of being invoked are those
  		that have been removed from the rcu_data structures
  		queues by rcu_do_batch(), but which have not yet been
  		invoked.
  
  	If there are no callbacks in a given one of the above states,
  	the corresponding character is replaced by ".".
  
  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+nci+ql is the number of callbacks that have
  	been registered in absence of CPU-hotplug activity.
  
  o	"nci" is the number of RCU callbacks that have been offloaded from
  	this CPU.  This will always be zero unless the kernel was built
  	with CONFIG_RCU_NOCB_CPU=y and the "rcu_nocbs=" kernel boot
  	parameter was specified.
  
  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 by this
  	CPU due to other CPUs going offline.  Note that ci+co-ca+ql is
  	the number of RCU callbacks registered on this CPU.
  
  
  Kernels compiled with CONFIG_RCU_BOOST=y display the following from
  /debug/rcu/rcu_preempt/rcudata:
  
    0!c=12865 g=12866 pq=1 qp=1 dt=83113/140000000000000/0 df=288 of=11 ql=0/0 qs=N... kt=0/O ktl=944 b=10 ci=60709 nci=0 co=748 ca=871
    1 c=14407 g=14408 pq=1 qp=0 dt=100679/140000000000000/0 df=378 of=7 ql=0/119 qs=NRW. kt=0/W ktl=9b6 b=10 ci=109740 nci=0 co=589 ca=485
    2 c=14407 g=14408 pq=1 qp=0 dt=105486/0/0 df=90 of=9 ql=0/89 qs=NRW. kt=0/W ktl=c0c b=10 ci=83113 nci=0 co=533 ca=490
    3 c=14407 g=14408 pq=1 qp=0 dt=107138/0/0 df=142 of=8 ql=0/188 qs=NRW. kt=0/W ktl=b96 b=10 ci=121114 nci=0 co=426 ca=290
    4 c=14405 g=14406 pq=1 qp=1 dt=50238/0/0 df=706 of=7 ql=0/0 qs=.... kt=0/W ktl=812 b=10 ci=34929 nci=0 co=643 ca=114
    5!c=14168 g=14169 pq=1 qp=0 dt=45465/140000000000000/0 df=161 of=11 ql=0/0 qs=N... kt=0/O ktl=b4d b=10 ci=47712 nci=0 co=677 ca=722
    6 c=14404 g=14405 pq=1 qp=0 dt=59454/0/0 df=94 of=6 ql=0/0 qs=.... kt=0/W ktl=e57 b=10 ci=55597 nci=0 co=701 ca=811
    7 c=14407 g=14408 pq=1 qp=1 dt=68850/0/0 df=31 of=8 ql=0/0 qs=.... kt=0/W ktl=14bd b=10 ci=77475 nci=0 co=508 ca=1042
  
  This is similar to the output discussed above, but contains the following
  additional fields:
  
  o	"kt" is the per-CPU kernel-thread state.  The digit preceding
  	the first slash is zero if there is no work pending and 1
  	otherwise.  The character between the first pair of slashes is
  	as follows:
  
  	"S"	The kernel thread is stopped, in other words, all
  		CPUs corresponding to this rcu_node structure are
  		offline.
  
  	"R"	The kernel thread is running.
  
  	"W"	The kernel thread is waiting because there is no work
  		for it to do.
  
  	"O"	The kernel thread is waiting because it has been
  		forced off of its designated CPU or because its
  		->cpus_allowed mask permits it to run on other than
  		its designated CPU.
  
  	"Y"	The kernel thread is yielding to avoid hogging CPU.
  
  	"?"	Unknown value, indicates a bug.
  
  	The number after the final slash is the CPU that the kthread
  	is actually running on.
  
  	This field is displayed only for CONFIG_RCU_BOOST kernels.
  
  o	"ktl" is the low-order 16 bits (in hexadecimal) of the count of
  	the number of times that this CPU's per-CPU kthread has gone
  	through its loop servicing invoke_rcu_cpu_kthread() requests.
  
  	This field is displayed only for CONFIG_RCU_BOOST kernels.
  
  
  The output of "cat rcu/rcu_preempt/rcuexp" looks as follows:
  
  s=21872 d=21872 w=0 tf=0 wd1=0 wd2=0 n=0 sc=21872 dt=21872 dl=0 dx=21872
  
  These fields are as follows:
  
  o	"s" is the starting sequence number.
  
  o	"d" is the ending sequence number.  When the starting and ending
  	numbers differ, there is an expedited grace period in progress.
  
  o	"w" is the number of times that the sequence numbers have been
  	in danger of wrapping.
  
  o	"tf" is the number of times that contention has resulted in a
  	failure to begin an expedited grace period.
  
  o	"wd1" and "wd2" are the number of times that an attempt to
  	start an expedited grace period found that someone else had
  	completed an expedited grace period that satisfies the
  	attempted request.  "Our work is done."
  
  o	"n" is number of times that contention was so great that
  	the request was demoted from an expedited grace period to
  	a normal grace period.
  
  o	"sc" is the number of times that the attempt to start a
  	new expedited grace period succeeded.
  
  o	"dt" is the number of times that we attempted to update
  	the "d" counter.
  
  o	"dl" is the number of times that we failed to update the "d"
  	counter.
  
  o	"dx" is the number of times that we succeeded in updating
  	the "d" counter.
  
  
  The output of "cat rcu/rcu_preempt/rcugp" looks as follows:
  
  completed=31249  gpnum=31250  age=1  max=18
  
  These 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
  	similarly 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, 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 are above), then an RCU grace
  	period is in progress.
  
  o	"age" is the number of jiffies that the current grace period
  	has extended for, or zero if there is no grace period currently
  	in effect.
  
  o	"max" is the age in jiffies of the longest-duration grace period
  	thus far.
  
  The output of "cat rcu/rcu_preempt/rcuhier" looks as follows:
  
  c=14407 g=14408 s=0 jfq=2 j=c863 nfqs=12040/nfqsng=0(12040) fqlh=1051 oqlen=0/0
  3/3 ..>. 0:7 ^0
  e/e ..>. 0:3 ^0    d/d ..>. 4:7 ^1
  
  The fields are as follows:
  
  o	"c" is exactly the same as "completed" under rcu/rcu_preempt/rcugp.
  
  o	"g" is exactly the same as "gpnum" under rcu/rcu_preempt/rcugp.
  
  o	"s" is the current state of the force_quiescent_state()
  	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 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
  	no longer happen due to grace-period processing being pushed
  	into a kthread.  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 "3/3 ..>. 0:7 ^0" represents one rcu_node
  	structure.  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, three, or even four
  	levels of rcu_node structures, depending on the relationship
  	between CONFIG_RCU_FANOUT, CONFIG_RCU_FANOUT_LEAF (possibly
  	adjusted using the rcu_fanout_leaf kernel boot parameter), and
  	CONFIG_NR_CPUS (possibly adjusted using the nr_cpu_ids count of
  	possible CPUs for the booting hardware).
  
  	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 ("e"
  		indicating CPUs 5, 6, and 7 in the example above).
  		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.
  
  	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.  CONFIG_TREE_RCU
  		builds of the kernel will always show "..>.".
  
  	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 example rcu_node structure shown above
  		has "0:7", indicating that it covers CPUs 0 through 7.
  
  	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 "d/d ..>. 4:7
  		^1" has a "1" in this position, indicating that it
  		corresponds to the "1" bit in the "3" shown in the
  		"3/3 ..>. 0:7 ^0" entry on the next level up.
  
  
  The output of "cat rcu/rcu_sched/rcu_pending" looks as follows:
  
    0!np=26111 qsp=29 rpq=5386 cbr=1 cng=570 gpc=3674 gps=577 nn=15903 ndw=0
    1!np=28913 qsp=35 rpq=6097 cbr=1 cng=448 gpc=3700 gps=554 nn=18113 ndw=0
    2!np=32740 qsp=37 rpq=6202 cbr=0 cng=476 gpc=4627 gps=546 nn=20889 ndw=0
    3 np=23679 qsp=22 rpq=5044 cbr=1 cng=415 gpc=3403 gps=347 nn=14469 ndw=0
    4!np=30714 qsp=4 rpq=5574 cbr=0 cng=528 gpc=3931 gps=639 nn=20042 ndw=0
    5 np=28910 qsp=2 rpq=5246 cbr=0 cng=428 gpc=4105 gps=709 nn=18422 ndw=0
    6!np=38648 qsp=5 rpq=7076 cbr=0 cng=840 gpc=4072 gps=961 nn=25699 ndw=0
    7 np=37275 qsp=2 rpq=6873 cbr=0 cng=868 gpc=3416 gps=971 nn=25147 ndw=0
  
  The fields are as follows:
  
  o	The leading number is the CPU number, with "!" indicating
  	an offline CPU.
  
  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	"ndw" is the number of times that a wakeup of an rcuo
  	callback-offload kthread had to be deferred in order to avoid
  	deadlock.
  
  o	"nn" is the number of times that this CPU needed nothing.
  
  
  The output of "cat rcu/rcuboost" looks as follows:
  
  0:3 tasks=.... kt=W ntb=0 neb=0 nnb=0 j=c864 bt=c894
      balk: nt=0 egt=4695 bt=0 nb=0 ny=56 nos=0
  4:7 tasks=.... kt=W ntb=0 neb=0 nnb=0 j=c864 bt=c894
      balk: nt=0 egt=6541 bt=0 nb=0 ny=126 nos=0
  
  This information is output only for rcu_preempt.  Each two-line entry
  corresponds to a leaf rcu_node structure.  The fields are as follows:
  
  o	"n:m" is the CPU-number range for the corresponding two-line
  	entry.  In the sample output above, the first entry covers
  	CPUs zero through three and the second entry covers CPUs four
  	through seven.
  
  o	"tasks=TNEB" gives the state of the various segments of the
  	rnp->blocked_tasks list:
  
  	"T"	This indicates that there are some tasks that blocked
  		while running on one of the corresponding CPUs while
  		in an RCU read-side critical section.
  
  	"N"	This indicates that some of the blocked tasks are preventing
  		the current normal (non-expedited) grace period from
  		completing.
  
  	"E"	This indicates that some of the blocked tasks are preventing
  		the current expedited grace period from completing.
  
  	"B"	This indicates that some of the blocked tasks are in
  		need of RCU priority boosting.
  
  	Each character is replaced with "." if the corresponding
  	condition does not hold.
  
  o	"kt" is the state of the RCU priority-boosting kernel
  	thread associated with the corresponding rcu_node structure.
  	The state can be one of the following:
  
  	"S"	The kernel thread is stopped, in other words, all
  		CPUs corresponding to this rcu_node structure are
  		offline.
  
  	"R"	The kernel thread is running.
  
  	"W"	The kernel thread is waiting because there is no work
  		for it to do.
  
  	"Y"	The kernel thread is yielding to avoid hogging CPU.
  
  	"?"	Unknown value, indicates a bug.
  
  o	"ntb" is the number of tasks boosted.
  
  o	"neb" is the number of tasks boosted in order to complete an
  	expedited grace period.
  
  o	"nnb" is the number of tasks boosted in order to complete a
  	normal (non-expedited) grace period.  When boosting a task
  	that was blocking both an expedited and a normal grace period,
  	it is counted against the expedited total above.
  
  o	"j" is the low-order 16 bits of the jiffies counter in
  	hexadecimal.
  
  o	"bt" is the low-order 16 bits of the value that the jiffies
  	counter will have when we next start boosting, assuming that
  	the current grace period does not end beforehand.  This is
  	also in hexadecimal.
  
  o	"balk: nt" counts the number of times we didn't boost (in
  	other words, we balked) even though it was time to boost because
  	there were no blocked tasks to boost.  This situation occurs
  	when there is one blocked task on one rcu_node structure and
  	none on some other rcu_node structure.
  
  o	"egt" counts the number of times we balked because although
  	there were blocked tasks, none of them were blocking the
  	current grace period, whether expedited or otherwise.
  
  o	"bt" counts the number of times we balked because boosting
  	had already been initiated for the current grace period.
  
  o	"nb" counts the number of times we balked because there
  	was at least one task blocking the current non-expedited grace
  	period that never had blocked.  If it is already running, it
  	just won't help to boost its priority!
  
  o	"ny" counts the number of times we balked because it was
  	not yet time to start boosting.
  
  o	"nos" counts the number of times we balked for other
  	reasons, e.g., the grace period ended first.
  
  
  CONFIG_TINY_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 and rcu_sched_ctrlblk.
  
  The output of "cat rcu/rcudata" is as follows:
  
  rcu_sched: qlen: 0
  rcu_bh: qlen: 0
  
  This is split into rcu_sched and rcu_bh sections.  The field is 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.