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Based on kernel version 4.3. Page generated on 2015-11-02 12:50 EST.

1	CONFIG_RCU_TRACE debugfs Files and Formats
2	
3	
4	The rcutree and rcutiny implementations of RCU provide debugfs trace
5	output that summarizes counters and state.  This information is useful for
6	debugging RCU itself, and can sometimes also help to debug abuses of RCU.
7	The following sections describe the debugfs files and formats, first
8	for rcutree and next for rcutiny.
9	
10	
11	CONFIG_TREE_RCU and CONFIG_PREEMPT_RCU debugfs Files and Formats
12	
13	These implementations of RCU provide several debugfs directories under the
14	top-level directory "rcu":
15	
16	rcu/rcu_bh
17	rcu/rcu_preempt
18	rcu/rcu_sched
19	
20	Each directory contains files for the corresponding flavor of RCU.
21	Note that rcu/rcu_preempt is only present for CONFIG_PREEMPT_RCU.
22	For CONFIG_TREE_RCU, the RCU flavor maps onto the RCU-sched flavor,
23	so that activity for both appears in rcu/rcu_sched.
24	
25	In addition, the following file appears in the top-level directory:
26	rcu/rcutorture.  This file displays rcutorture test progress.  The output
27	of "cat rcu/rcutorture" looks as follows:
28	
29	rcutorture test sequence: 0 (test in progress)
30	rcutorture update version number: 615
31	
32	The first line shows the number of rcutorture tests that have completed
33	since boot.  If a test is currently running, the "(test in progress)"
34	string will appear as shown above.  The second line shows the number of
35	update cycles that the current test has started, or zero if there is
36	no test in progress.
37	
38	
39	Within each flavor directory (rcu/rcu_bh, rcu/rcu_sched, and possibly
40	also rcu/rcu_preempt) the following files will be present:
41	
42	rcudata:
43		Displays fields in struct rcu_data.
44	rcuexp:
45		Displays statistics for expedited grace periods.
46	rcugp:
47		Displays grace-period counters.
48	rcuhier:
49		Displays the struct rcu_node hierarchy.
50	rcu_pending:
51		Displays counts of the reasons rcu_pending() decided that RCU had
52		work to do.
53	rcuboost:
54		Displays RCU boosting statistics.  Only present if
55		CONFIG_RCU_BOOST=y.
56	
57	The output of "cat rcu/rcu_preempt/rcudata" looks as follows:
58	
59	  0!c=30455 g=30456 pq=1/0 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
60	  1!c=30719 g=30720 pq=1/0 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
61	  2!c=30150 g=30151 pq=1/1 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
62	  3 c=31249 g=31250 pq=1/1 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
63	  4!c=29502 g=29503 pq=1/0 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
64	  5 c=31201 g=31202 pq=1/0 qp=1 dt=70422/0/0 df=535 of=7 ql=0/0 qs=.... b=10 ci=58500 nci=0 co=764 ca=698
65	  6!c=30253 g=30254 pq=1/0 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
66	  7 c=31178 g=31178 pq=1/0 qp=0 dt=91536/0/0 df=547 of=4 ql=0/0 qs=.... b=10 ci=109819 nci=0 co=1115 ca=969
67	
68	This file has one line per CPU, or eight for this 8-CPU system.
69	The fields are as follows:
70	
71	o	The number at the beginning of each line is the CPU number.
72		CPUs numbers followed by an exclamation mark are offline,
73		but have been online at least once since boot.	There will be
74		no output for CPUs that have never been online, which can be
75		a good thing in the surprisingly common case where NR_CPUS is
76		substantially larger than the number of actual CPUs.
77	
78	o	"c" is the count of grace periods that this CPU believes have
79		completed.  Offlined CPUs and CPUs in dynticks idle mode may lag
80		quite a ways behind, for example, CPU 4 under "rcu_sched" above,
81		which has been offline through 16 RCU grace periods.  It is not
82		unusual to see offline CPUs lagging by thousands of grace periods.
83		Note that although the grace-period number is an unsigned long,
84		it is printed out as a signed long to allow more human-friendly
85		representation near boot time.
86	
87	o	"g" is the count of grace periods that this CPU believes have
88		started.  Again, offlined CPUs and CPUs in dynticks idle mode
89		may lag behind.  If the "c" and "g" values are equal, this CPU
90		has already reported a quiescent state for the last RCU grace
91		period that it is aware of, otherwise, the CPU believes that it
92		owes RCU a quiescent state.
93	
94	o	"pq" indicates that this CPU has passed through a quiescent state
95		for the current grace period.  It is possible for "pq" to be
96		"1" and "c" different than "g", which indicates that although
97		the CPU has passed through a quiescent state, either (1) this
98		CPU has not yet reported that fact, (2) some other CPU has not
99		yet reported for this grace period, or (3) both.
100	
101	o	"qp" indicates that RCU still expects a quiescent state from
102		this CPU.  Offlined CPUs and CPUs in dyntick idle mode might
103		well have qp=1, which is OK: RCU is still ignoring them.
104	
105	o	"dt" is the current value of the dyntick counter that is incremented
106		when entering or leaving idle, either due to a context switch or
107		due to an interrupt.  This number is even if the CPU is in idle
108		from RCU's viewpoint and odd otherwise.  The number after the
109		first "/" is the interrupt nesting depth when in idle state,
110		or a large number added to the interrupt-nesting depth when
111		running a non-idle task.  Some architectures do not accurately
112		count interrupt nesting when running in non-idle kernel context,
113		which can result in interesting anomalies such as negative
114		interrupt-nesting levels.  The number after the second "/"
115		is the NMI nesting depth.
116	
117	o	"df" is the number of times that some other CPU has forced a
118		quiescent state on behalf of this CPU due to this CPU being in
119		idle state.
120	
121	o	"of" is the number of times that some other CPU has forced a
122		quiescent state on behalf of this CPU due to this CPU being
123		offline.  In a perfect world, this might never happen, but it
124		turns out that offlining and onlining a CPU can take several grace
125		periods, and so there is likely to be an extended period of time
126		when RCU believes that the CPU is online when it really is not.
127		Please note that erring in the other direction (RCU believing a
128		CPU is offline when it is really alive and kicking) is a fatal
129		error, so it makes sense to err conservatively.
130	
131	o	"ql" is the number of RCU callbacks currently residing on
132		this CPU.  The first number is the number of "lazy" callbacks
133		that are known to RCU to only be freeing memory, and the number
134		after the "/" is the total number of callbacks, lazy or not.
135		These counters count callbacks regardless of what phase of
136		grace-period processing that they are in (new, waiting for
137		grace period to start, waiting for grace period to end, ready
138		to invoke).
139	
140	o	"qs" gives an indication of the state of the callback queue
141		with four characters:
142	
143		"N"	Indicates that there are callbacks queued that are not
144			ready to be handled by the next grace period, and thus
145			will be handled by the grace period following the next
146			one.
147	
148		"R"	Indicates that there are callbacks queued that are
149			ready to be handled by the next grace period.
150	
151		"W"	Indicates that there are callbacks queued that are
152			waiting on the current grace period.
153	
154		"D"	Indicates that there are callbacks queued that have
155			already been handled by a prior grace period, and are
156			thus waiting to be invoked.  Note that callbacks in
157			the process of being invoked are not counted here.
158			Callbacks in the process of being invoked are those
159			that have been removed from the rcu_data structures
160			queues by rcu_do_batch(), but which have not yet been
161			invoked.
162	
163		If there are no callbacks in a given one of the above states,
164		the corresponding character is replaced by ".".
165	
166	o	"b" is the batch limit for this CPU.  If more than this number
167		of RCU callbacks is ready to invoke, then the remainder will
168		be deferred.
169	
170	o	"ci" is the number of RCU callbacks that have been invoked for
171		this CPU.  Note that ci+nci+ql is the number of callbacks that have
172		been registered in absence of CPU-hotplug activity.
173	
174	o	"nci" is the number of RCU callbacks that have been offloaded from
175		this CPU.  This will always be zero unless the kernel was built
176		with CONFIG_RCU_NOCB_CPU=y and the "rcu_nocbs=" kernel boot
177		parameter was specified.
178	
179	o	"co" is the number of RCU callbacks that have been orphaned due to
180		this CPU going offline.  These orphaned callbacks have been moved
181		to an arbitrarily chosen online CPU.
182	
183	o	"ca" is the number of RCU callbacks that have been adopted by this
184		CPU due to other CPUs going offline.  Note that ci+co-ca+ql is
185		the number of RCU callbacks registered on this CPU.
186	
187	
188	Kernels compiled with CONFIG_RCU_BOOST=y display the following from
189	/debug/rcu/rcu_preempt/rcudata:
190	
191	  0!c=12865 g=12866 pq=1/0 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
192	  1 c=14407 g=14408 pq=1/0 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
193	  2 c=14407 g=14408 pq=1/0 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
194	  3 c=14407 g=14408 pq=1/0 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
195	  4 c=14405 g=14406 pq=1/0 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
196	  5!c=14168 g=14169 pq=1/0 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
197	  6 c=14404 g=14405 pq=1/0 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
198	  7 c=14407 g=14408 pq=1/0 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
199	
200	This is similar to the output discussed above, but contains the following
201	additional fields:
202	
203	o	"kt" is the per-CPU kernel-thread state.  The digit preceding
204		the first slash is zero if there is no work pending and 1
205		otherwise.  The character between the first pair of slashes is
206		as follows:
207	
208		"S"	The kernel thread is stopped, in other words, all
209			CPUs corresponding to this rcu_node structure are
210			offline.
211	
212		"R"	The kernel thread is running.
213	
214		"W"	The kernel thread is waiting because there is no work
215			for it to do.
216	
217		"O"	The kernel thread is waiting because it has been
218			forced off of its designated CPU or because its
219			->cpus_allowed mask permits it to run on other than
220			its designated CPU.
221	
222		"Y"	The kernel thread is yielding to avoid hogging CPU.
223	
224		"?"	Unknown value, indicates a bug.
225	
226		The number after the final slash is the CPU that the kthread
227		is actually running on.
228	
229		This field is displayed only for CONFIG_RCU_BOOST kernels.
230	
231	o	"ktl" is the low-order 16 bits (in hexadecimal) of the count of
232		the number of times that this CPU's per-CPU kthread has gone
233		through its loop servicing invoke_rcu_cpu_kthread() requests.
234	
235		This field is displayed only for CONFIG_RCU_BOOST kernels.
236	
237	
238	The output of "cat rcu/rcu_preempt/rcuexp" looks as follows:
239	
240	s=21872 wd0=0 wd1=0 wd2=0 wd3=5 n=0 enq=0 sc=21872
241	
242	These fields are as follows:
243	
244	o	"s" is the sequence number, with an odd number indicating that
245		an expedited grace period is in progress.
246	
247	o	"wd0", "wd1", "wd2", and "wd3" are the number of times that an
248		attempt to start an expedited grace period found that someone
249		else had completed an expedited grace period that satisfies the
250		attempted request.  "Our work is done."
251	
252	o	"n" is number of times that a concurrent CPU-hotplug operation
253		forced a fallback to a normal grace period.
254	
255	o	"enq" is the number of quiescent states still outstanding.
256	
257	o	"sc" is the number of times that the attempt to start a
258		new expedited grace period succeeded.
259	
260	
261	The output of "cat rcu/rcu_preempt/rcugp" looks as follows:
262	
263	completed=31249  gpnum=31250  age=1  max=18
264	
265	These fields are taken from the rcu_state structure, and are as follows:
266	
267	o	"completed" is the number of grace periods that have completed.
268		It is comparable to the "c" field from rcu/rcudata in that a
269		CPU whose "c" field matches the value of "completed" is aware
270		that the corresponding RCU grace period has completed.
271	
272	o	"gpnum" is the number of grace periods that have started.  It is
273		similarly comparable to the "g" field from rcu/rcudata in that
274		a CPU whose "g" field matches the value of "gpnum" is aware that
275		the corresponding RCU grace period has started.
276	
277		If these two fields are equal, then there is no grace period
278		in progress, in other words, RCU is idle.  On the other hand,
279		if the two fields differ (as they are above), then an RCU grace
280		period is in progress.
281	
282	o	"age" is the number of jiffies that the current grace period
283		has extended for, or zero if there is no grace period currently
284		in effect.
285	
286	o	"max" is the age in jiffies of the longest-duration grace period
287		thus far.
288	
289	The output of "cat rcu/rcu_preempt/rcuhier" looks as follows:
290	
291	c=14407 g=14408 s=0 jfq=2 j=c863 nfqs=12040/nfqsng=0(12040) fqlh=1051 oqlen=0/0
292	3/3 ..>. 0:7 ^0
293	e/e ..>. 0:3 ^0    d/d ..>. 4:7 ^1
294	
295	The fields are as follows:
296	
297	o	"c" is exactly the same as "completed" under rcu/rcu_preempt/rcugp.
298	
299	o	"g" is exactly the same as "gpnum" under rcu/rcu_preempt/rcugp.
300	
301	o	"s" is the current state of the force_quiescent_state()
302		state machine.
303	
304	o	"jfq" is the number of jiffies remaining for this grace period
305		before force_quiescent_state() is invoked to help push things
306		along.	Note that CPUs in idle mode throughout the grace period
307		will not report on their own, but rather must be check by some
308		other CPU via force_quiescent_state().
309	
310	o	"j" is the low-order four hex digits of the jiffies counter.
311		Yes, Paul did run into a number of problems that turned out to
312		be due to the jiffies counter no longer counting.  Why do you ask?
313	
314	o	"nfqs" is the number of calls to force_quiescent_state() since
315		boot.
316	
317	o	"nfqsng" is the number of useless calls to force_quiescent_state(),
318		where there wasn't actually a grace period active.  This can
319		no longer happen due to grace-period processing being pushed
320		into a kthread.  The number in parentheses is the difference
321		between "nfqs" and "nfqsng", or the number of times that
322		force_quiescent_state() actually did some real work.
323	
324	o	"fqlh" is the number of calls to force_quiescent_state() that
325		exited immediately (without even being counted in nfqs above)
326		due to contention on ->fqslock.
327	
328	o	Each element of the form "3/3 ..>. 0:7 ^0" represents one rcu_node
329		structure.  Each line represents one level of the hierarchy,
330		from root to leaves.  It is best to think of the rcu_data
331		structures as forming yet another level after the leaves.
332		Note that there might be either one, two, three, or even four
333		levels of rcu_node structures, depending on the relationship
334		between CONFIG_RCU_FANOUT, CONFIG_RCU_FANOUT_LEAF (possibly
335		adjusted using the rcu_fanout_leaf kernel boot parameter), and
336		CONFIG_NR_CPUS (possibly adjusted using the nr_cpu_ids count of
337		possible CPUs for the booting hardware).
338	
339		o	The numbers separated by the "/" are the qsmask followed
340			by the qsmaskinit.  The qsmask will have one bit
341			set for each entity in the next lower level that has
342			not yet checked in for the current grace period ("e"
343			indicating CPUs 5, 6, and 7 in the example above).
344			The qsmaskinit will have one bit for each entity that is
345			currently expected to check in during each grace period.
346			The value of qsmaskinit is assigned to that of qsmask
347			at the beginning of each grace period.
348	
349		o	The characters separated by the ">" indicate the state
350			of the blocked-tasks lists.  A "G" preceding the ">"
351			indicates that at least one task blocked in an RCU
352			read-side critical section blocks the current grace
353			period, while a "E" preceding the ">" indicates that
354			at least one task blocked in an RCU read-side critical
355			section blocks the current expedited grace period.
356			A "T" character following the ">" indicates that at
357			least one task is blocked within an RCU read-side
358			critical section, regardless of whether any current
359			grace period (expedited or normal) is inconvenienced.
360			A "." character appears if the corresponding condition
361			does not hold, so that "..>." indicates that no tasks
362			are blocked.  In contrast, "GE>T" indicates maximal
363			inconvenience from blocked tasks.  CONFIG_TREE_RCU
364			builds of the kernel will always show "..>.".
365	
366		o	The numbers separated by the ":" are the range of CPUs
367			served by this struct rcu_node.  This can be helpful
368			in working out how the hierarchy is wired together.
369	
370			For example, the example rcu_node structure shown above
371			has "0:7", indicating that it covers CPUs 0 through 7.
372	
373		o	The number after the "^" indicates the bit in the
374			next higher level rcu_node structure that this rcu_node
375			structure corresponds to.  For example, the "d/d ..>. 4:7
376			^1" has a "1" in this position, indicating that it
377			corresponds to the "1" bit in the "3" shown in the
378			"3/3 ..>. 0:7 ^0" entry on the next level up.
379	
380	
381	The output of "cat rcu/rcu_sched/rcu_pending" looks as follows:
382	
383	  0!np=26111 qsp=29 rpq=5386 cbr=1 cng=570 gpc=3674 gps=577 nn=15903 ndw=0
384	  1!np=28913 qsp=35 rpq=6097 cbr=1 cng=448 gpc=3700 gps=554 nn=18113 ndw=0
385	  2!np=32740 qsp=37 rpq=6202 cbr=0 cng=476 gpc=4627 gps=546 nn=20889 ndw=0
386	  3 np=23679 qsp=22 rpq=5044 cbr=1 cng=415 gpc=3403 gps=347 nn=14469 ndw=0
387	  4!np=30714 qsp=4 rpq=5574 cbr=0 cng=528 gpc=3931 gps=639 nn=20042 ndw=0
388	  5 np=28910 qsp=2 rpq=5246 cbr=0 cng=428 gpc=4105 gps=709 nn=18422 ndw=0
389	  6!np=38648 qsp=5 rpq=7076 cbr=0 cng=840 gpc=4072 gps=961 nn=25699 ndw=0
390	  7 np=37275 qsp=2 rpq=6873 cbr=0 cng=868 gpc=3416 gps=971 nn=25147 ndw=0
391	
392	The fields are as follows:
393	
394	o	The leading number is the CPU number, with "!" indicating
395		an offline CPU.
396	
397	o	"np" is the number of times that __rcu_pending() has been invoked
398		for the corresponding flavor of RCU.
399	
400	o	"qsp" is the number of times that the RCU was waiting for a
401		quiescent state from this CPU.
402	
403	o	"rpq" is the number of times that the CPU had passed through
404		a quiescent state, but not yet reported it to RCU.
405	
406	o	"cbr" is the number of times that this CPU had RCU callbacks
407		that had passed through a grace period, and were thus ready
408		to be invoked.
409	
410	o	"cng" is the number of times that this CPU needed another
411		grace period while RCU was idle.
412	
413	o	"gpc" is the number of times that an old grace period had
414		completed, but this CPU was not yet aware of it.
415	
416	o	"gps" is the number of times that a new grace period had started,
417		but this CPU was not yet aware of it.
418	
419	o	"ndw" is the number of times that a wakeup of an rcuo
420		callback-offload kthread had to be deferred in order to avoid
421		deadlock.
422	
423	o	"nn" is the number of times that this CPU needed nothing.
424	
425	
426	The output of "cat rcu/rcuboost" looks as follows:
427	
428	0:3 tasks=.... kt=W ntb=0 neb=0 nnb=0 j=c864 bt=c894
429	    balk: nt=0 egt=4695 bt=0 nb=0 ny=56 nos=0
430	4:7 tasks=.... kt=W ntb=0 neb=0 nnb=0 j=c864 bt=c894
431	    balk: nt=0 egt=6541 bt=0 nb=0 ny=126 nos=0
432	
433	This information is output only for rcu_preempt.  Each two-line entry
434	corresponds to a leaf rcu_node structure.  The fields are as follows:
435	
436	o	"n:m" is the CPU-number range for the corresponding two-line
437		entry.  In the sample output above, the first entry covers
438		CPUs zero through three and the second entry covers CPUs four
439		through seven.
440	
441	o	"tasks=TNEB" gives the state of the various segments of the
442		rnp->blocked_tasks list:
443	
444		"T"	This indicates that there are some tasks that blocked
445			while running on one of the corresponding CPUs while
446			in an RCU read-side critical section.
447	
448		"N"	This indicates that some of the blocked tasks are preventing
449			the current normal (non-expedited) grace period from
450			completing.
451	
452		"E"	This indicates that some of the blocked tasks are preventing
453			the current expedited grace period from completing.
454	
455		"B"	This indicates that some of the blocked tasks are in
456			need of RCU priority boosting.
457	
458		Each character is replaced with "." if the corresponding
459		condition does not hold.
460	
461	o	"kt" is the state of the RCU priority-boosting kernel
462		thread associated with the corresponding rcu_node structure.
463		The state can be one of the following:
464	
465		"S"	The kernel thread is stopped, in other words, all
466			CPUs corresponding to this rcu_node structure are
467			offline.
468	
469		"R"	The kernel thread is running.
470	
471		"W"	The kernel thread is waiting because there is no work
472			for it to do.
473	
474		"Y"	The kernel thread is yielding to avoid hogging CPU.
475	
476		"?"	Unknown value, indicates a bug.
477	
478	o	"ntb" is the number of tasks boosted.
479	
480	o	"neb" is the number of tasks boosted in order to complete an
481		expedited grace period.
482	
483	o	"nnb" is the number of tasks boosted in order to complete a
484		normal (non-expedited) grace period.  When boosting a task
485		that was blocking both an expedited and a normal grace period,
486		it is counted against the expedited total above.
487	
488	o	"j" is the low-order 16 bits of the jiffies counter in
489		hexadecimal.
490	
491	o	"bt" is the low-order 16 bits of the value that the jiffies
492		counter will have when we next start boosting, assuming that
493		the current grace period does not end beforehand.  This is
494		also in hexadecimal.
495	
496	o	"balk: nt" counts the number of times we didn't boost (in
497		other words, we balked) even though it was time to boost because
498		there were no blocked tasks to boost.  This situation occurs
499		when there is one blocked task on one rcu_node structure and
500		none on some other rcu_node structure.
501	
502	o	"egt" counts the number of times we balked because although
503		there were blocked tasks, none of them were blocking the
504		current grace period, whether expedited or otherwise.
505	
506	o	"bt" counts the number of times we balked because boosting
507		had already been initiated for the current grace period.
508	
509	o	"nb" counts the number of times we balked because there
510		was at least one task blocking the current non-expedited grace
511		period that never had blocked.  If it is already running, it
512		just won't help to boost its priority!
513	
514	o	"ny" counts the number of times we balked because it was
515		not yet time to start boosting.
516	
517	o	"nos" counts the number of times we balked for other
518		reasons, e.g., the grace period ended first.
519	
520	
521	CONFIG_TINY_RCU debugfs Files and Formats
522	
523	These implementations of RCU provides a single debugfs file under the
524	top-level directory RCU, namely rcu/rcudata, which displays fields in
525	rcu_bh_ctrlblk and rcu_sched_ctrlblk.
526	
527	The output of "cat rcu/rcudata" is as follows:
528	
529	rcu_sched: qlen: 0
530	rcu_bh: qlen: 0
531	
532	This is split into rcu_sched and rcu_bh sections.  The field is as
533	follows:
534	
535	o	"qlen" is the number of RCU callbacks currently waiting either
536		for an RCU grace period or waiting to be invoked.  This is the
537		only field present for rcu_sched and rcu_bh, due to the
538		short-circuiting of grace period in those two cases.
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