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1CONFIG_RCU_TRACE debugfs Files and Formats
2
3
4The rcupreempt and rcutree implementations of RCU provide debugfs trace
5output that summarizes counters and state. This information is useful for
6debugging RCU itself, and can sometimes also help to debug abuses of RCU.
7Note that the rcuclassic implementation of RCU does not provide debugfs
8trace output.
9
10The following sections describe the debugfs files and formats for
11preemptable RCU (rcupreempt) and hierarchical RCU (rcutree).
12
13
14Preemptable RCU debugfs Files and Formats
15
16This implementation of RCU provides three debugfs files under the
17top-level directory RCU: rcu/rcuctrs (which displays the per-CPU
18counters used by preemptable RCU) rcu/rcugp (which displays grace-period
19counters), and rcu/rcustats (which internal counters for debugging RCU).
20
21The output of "cat rcu/rcuctrs" looks as follows:
22
23CPU last cur F M
24 0 5 -5 0 0
25 1 -1 0 0 0
26 2 0 1 0 0
27 3 0 1 0 0
28 4 0 1 0 0
29 5 0 1 0 0
30 6 0 2 0 0
31 7 0 -1 0 0
32 8 0 1 0 0
33ggp = 26226, state = waitzero
34
35The per-CPU fields are as follows:
36
37o "CPU" gives the CPU number. Offline CPUs are not displayed.
38
39o "last" gives the value of the counter that is being decremented
40 for the current grace period phase. In the example above,
41 the counters sum to 4, indicating that there are still four
42 RCU read-side critical sections still running that started
43 before the last counter flip.
44
45o "cur" gives the value of the counter that is currently being
46 both incremented (by rcu_read_lock()) and decremented (by
47 rcu_read_unlock()). In the example above, the counters sum to
48 1, indicating that there is only one RCU read-side critical section
49 still running that started after the last counter flip.
50
51o "F" indicates whether RCU is waiting for this CPU to acknowledge
52 a counter flip. In the above example, RCU is not waiting on any,
53 which is consistent with the state being "waitzero" rather than
54 "waitack".
55
56o "M" indicates whether RCU is waiting for this CPU to execute a
57 memory barrier. In the above example, RCU is not waiting on any,
58 which is consistent with the state being "waitzero" rather than
59 "waitmb".
60
61o "ggp" is the global grace-period counter.
62
63o "state" is the RCU state, which can be one of the following:
64
65 o "idle": there is no grace period in progress.
66
67 o "waitack": RCU just incremented the global grace-period
68 counter, which has the effect of reversing the roles of
69 the "last" and "cur" counters above, and is waiting for
70 all the CPUs to acknowledge the flip. Once the flip has
71 been acknowledged, CPUs will no longer be incrementing
72 what are now the "last" counters, so that their sum will
73 decrease monotonically down to zero.
74
75 o "waitzero": RCU is waiting for the sum of the "last" counters
76 to decrease to zero.
77
78 o "waitmb": RCU is waiting for each CPU to execute a memory
79 barrier, which ensures that instructions from a given CPU's
80 last RCU read-side critical section cannot be reordered
81 with instructions following the memory-barrier instruction.
82
83The output of "cat rcu/rcugp" looks as follows:
84
85oldggp=48870 newggp=48873
86
87Note that reading from this file provokes a synchronize_rcu(). The
88"oldggp" value is that of "ggp" from rcu/rcuctrs above, taken before
89executing the synchronize_rcu(), and the "newggp" value is also the
90"ggp" value, but taken after the synchronize_rcu() command returns.
91
92
93The output of "cat rcu/rcugp" looks as follows:
94
95na=1337955 nl=40 wa=1337915 wl=44 da=1337871 dl=0 dr=1337871 di=1337871
961=50989 e1=6138 i1=49722 ie1=82 g1=49640 a1=315203 ae1=265563 a2=49640
97z1=1401244 ze1=1351605 z2=49639 m1=5661253 me1=5611614 m2=49639
98
99These are counters tracking internal preemptable-RCU events, however,
100some of them may be useful for debugging algorithms using RCU. In
101particular, the "nl", "wl", and "dl" values track the number of RCU
102callbacks in various states. The fields are as follows:
103
104o "na" is the total number of RCU callbacks that have been enqueued
105 since boot.
106
107o "nl" is the number of RCU callbacks waiting for the previous
108 grace period to end so that they can start waiting on the next
109 grace period.
110
111o "wa" is the total number of RCU callbacks that have started waiting
112 for a grace period since boot. "na" should be roughly equal to
113 "nl" plus "wa".
114
115o "wl" is the number of RCU callbacks currently waiting for their
116 grace period to end.
117
118o "da" is the total number of RCU callbacks whose grace periods
119 have completed since boot. "wa" should be roughly equal to
120 "wl" plus "da".
121
122o "dr" is the total number of RCU callbacks that have been removed
123 from the list of callbacks ready to invoke. "dr" should be roughly
124 equal to "da".
125
126o "di" is the total number of RCU callbacks that have been invoked
127 since boot. "di" should be roughly equal to "da", though some
128 early versions of preemptable RCU had a bug so that only the
129 last CPU's count of invocations was displayed, rather than the
130 sum of all CPU's counts.
131
132o "1" is the number of calls to rcu_try_flip(). This should be
133 roughly equal to the sum of "e1", "i1", "a1", "z1", and "m1"
134 described below. In other words, the number of times that
135 the state machine is visited should be equal to the sum of the
136 number of times that each state is visited plus the number of
137 times that the state-machine lock acquisition failed.
138
139o "e1" is the number of times that rcu_try_flip() was unable to
140 acquire the fliplock.
141
142o "i1" is the number of calls to rcu_try_flip_idle().
143
144o "ie1" is the number of times rcu_try_flip_idle() exited early
145 due to the calling CPU having no work for RCU.
146
147o "g1" is the number of times that rcu_try_flip_idle() decided
148 to start a new grace period. "i1" should be roughly equal to
149 "ie1" plus "g1".
150
151o "a1" is the number of calls to rcu_try_flip_waitack().
152
153o "ae1" is the number of times that rcu_try_flip_waitack() found
154 that at least one CPU had not yet acknowledge the new grace period
155 (AKA "counter flip").
156
157o "a2" is the number of time rcu_try_flip_waitack() found that
158 all CPUs had acknowledged. "a1" should be roughly equal to
159 "ae1" plus "a2". (This particular output was collected on
160 a 128-CPU machine, hence the smaller-than-usual fraction of
161 calls to rcu_try_flip_waitack() finding all CPUs having already
162 acknowledged.)
163
164o "z1" is the number of calls to rcu_try_flip_waitzero().
165
166o "ze1" is the number of times that rcu_try_flip_waitzero() found
167 that not all of the old RCU read-side critical sections had
168 completed.
169
170o "z2" is the number of times that rcu_try_flip_waitzero() finds
171 the sum of the counters equal to zero, in other words, that
172 all of the old RCU read-side critical sections had completed.
173 The value of "z1" should be roughly equal to "ze1" plus
174 "z2".
175
176o "m1" is the number of calls to rcu_try_flip_waitmb().
177
178o "me1" is the number of times that rcu_try_flip_waitmb() finds
179 that at least one CPU has not yet executed a memory barrier.
180
181o "m2" is the number of times that rcu_try_flip_waitmb() finds that
182 all CPUs have executed a memory barrier.
183
184
185Hierarchical RCU debugfs Files and Formats
186
187This implementation of RCU provides three debugfs files under the
188top-level directory RCU: rcu/rcudata (which displays fields in struct
189rcu_data), rcu/rcugp (which displays grace-period counters), and
190rcu/rcuhier (which displays the struct rcu_node hierarchy).
191
192The output of "cat rcu/rcudata" looks as follows:
193
194rcu:
195 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
196 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
197 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
198 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
199 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
200 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
201 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
202 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
203rcu_bh:
204 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
205 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
206 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
207 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
208 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
209 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
210 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
211 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
212
213The first section lists the rcu_data structures for rcu, the second for
214rcu_bh. Each section has one line per CPU, or eight for this 8-CPU system.
215The fields are as follows:
216
217o The number at the beginning of each line is the CPU number.
218 CPUs numbers followed by an exclamation mark are offline,
219 but have been online at least once since boot. There will be
220 no output for CPUs that have never been online, which can be
221 a good thing in the surprisingly common case where NR_CPUS is
222 substantially larger than the number of actual CPUs.
223
224o "c" is the count of grace periods that this CPU believes have
225 completed. CPUs in dynticks idle mode may lag quite a ways
226 behind, for example, CPU 4 under "rcu" above, which has slept
227 through the past 25 RCU grace periods. It is not unusual to
228 see CPUs lagging by thousands of grace periods.
229
230o "g" is the count of grace periods that this CPU believes have
231 started. Again, CPUs in dynticks idle mode may lag behind.
232 If the "c" and "g" values are equal, this CPU has already
233 reported a quiescent state for the last RCU grace period that
234 it is aware of, otherwise, the CPU believes that it owes RCU a
235 quiescent state.
236
237o "pq" indicates that this CPU has passed through a quiescent state
238 for the current grace period. It is possible for "pq" to be
239 "1" and "c" different than "g", which indicates that although
240 the CPU has passed through a quiescent state, either (1) this
241 CPU has not yet reported that fact, (2) some other CPU has not
242 yet reported for this grace period, or (3) both.
243
244o "pqc" indicates which grace period the last-observed quiescent
245 state for this CPU corresponds to. This is important for handling
246 the race between CPU 0 reporting an extended dynticks-idle
247 quiescent state for CPU 1 and CPU 1 suddenly waking up and
248 reporting its own quiescent state. If CPU 1 was the last CPU
249 for the current grace period, then the CPU that loses this race
250 will attempt to incorrectly mark CPU 1 as having checked in for
251 the next grace period!
252
253o "qp" indicates that RCU still expects a quiescent state from
254 this CPU.
255
256o "rpfq" is the number of rcu_pending() calls on this CPU required
257 to induce this CPU to invoke force_quiescent_state().
258
259o "rp" is low-order four hex digits of the count of how many times
260 rcu_pending() has been invoked on this CPU.
261
262o "dt" is the current value of the dyntick counter that is incremented
263 when entering or leaving dynticks idle state, either by the
264 scheduler or by irq. The number after the "/" is the interrupt
265 nesting depth when in dyntick-idle state, or one greater than
266 the interrupt-nesting depth otherwise.
267
268 This field is displayed only for CONFIG_NO_HZ kernels.
269
270o "dn" is the current value of the dyntick counter that is incremented
271 when entering or leaving dynticks idle state via NMI. If both
272 the "dt" and "dn" values are even, then this CPU is in dynticks
273 idle mode and may be ignored by RCU. If either of these two
274 counters is odd, then RCU must be alert to the possibility of
275 an RCU read-side critical section running on this CPU.
276
277 This field is displayed only for CONFIG_NO_HZ kernels.
278
279o "df" is the number of times that some other CPU has forced a
280 quiescent state on behalf of this CPU due to this CPU being in
281 dynticks-idle state.
282
283 This field is displayed only for CONFIG_NO_HZ kernels.
284
285o "of" is the number of times that some other CPU has forced a
286 quiescent state on behalf of this CPU due to this CPU being
287 offline. In a perfect world, this might neve happen, but it
288 turns out that offlining and onlining a CPU can take several grace
289 periods, and so there is likely to be an extended period of time
290 when RCU believes that the CPU is online when it really is not.
291 Please note that erring in the other direction (RCU believing a
292 CPU is offline when it is really alive and kicking) is a fatal
293 error, so it makes sense to err conservatively.
294
295o "ri" is the number of times that RCU has seen fit to send a
296 reschedule IPI to this CPU in order to get it to report a
297 quiescent state.
298
299o "ql" is the number of RCU callbacks currently residing on
300 this CPU. This is the total number of callbacks, regardless
301 of what state they are in (new, waiting for grace period to
302 start, waiting for grace period to end, ready to invoke).
303
304o "b" is the batch limit for this CPU. If more than this number
305 of RCU callbacks is ready to invoke, then the remainder will
306 be deferred.
307
308
309The output of "cat rcu/rcugp" looks as follows:
310
311rcu: completed=33062 gpnum=33063
312rcu_bh: completed=464 gpnum=464
313
314Again, this output is for both "rcu" and "rcu_bh". The fields are
315taken from the rcu_state structure, and are as follows:
316
317o "completed" is the number of grace periods that have completed.
318 It is comparable to the "c" field from rcu/rcudata in that a
319 CPU whose "c" field matches the value of "completed" is aware
320 that the corresponding RCU grace period has completed.
321
322o "gpnum" is the number of grace periods that have started. It is
323 comparable to the "g" field from rcu/rcudata in that a CPU
324 whose "g" field matches the value of "gpnum" is aware that the
325 corresponding RCU grace period has started.
326
327 If these two fields are equal (as they are for "rcu_bh" above),
328 then there is no grace period in progress, in other words, RCU
329 is idle. On the other hand, if the two fields differ (as they
330 do for "rcu" above), then an RCU grace period is in progress.
331
332
333The output of "cat rcu/rcuhier" looks as follows, with very long lines:
334
335c=6902 g=6903 s=2 jfq=3 j=72c7 nfqs=13142/nfqsng=0(13142) fqlh=6
3361/1 0:127 ^0
3373/3 0:35 ^0 0/0 36:71 ^1 0/0 72:107 ^2 0/0 108:127 ^3
3383/3f 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
339rcu_bh:
340c=-226 g=-226 s=1 jfq=-5701 j=72c7 nfqs=88/nfqsng=0(88) fqlh=0
3410/1 0:127 ^0
3420/3 0:35 ^0 0/0 36:71 ^1 0/0 72:107 ^2 0/0 108:127 ^3
3430/3f 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
344
345This is once again split into "rcu" and "rcu_bh" portions. The fields are
346as follows:
347
348o "c" is exactly the same as "completed" under rcu/rcugp.
349
350o "g" is exactly the same as "gpnum" under rcu/rcugp.
351
352o "s" is the "signaled" state that drives force_quiescent_state()'s
353 state machine.
354
355o "jfq" is the number of jiffies remaining for this grace period
356 before force_quiescent_state() is invoked to help push things
357 along. Note that CPUs in dyntick-idle mode thoughout the grace
358 period will not report on their own, but rather must be check by
359 some other CPU via force_quiescent_state().
360
361o "j" is the low-order four hex digits of the jiffies counter.
362 Yes, Paul did run into a number of problems that turned out to
363 be due to the jiffies counter no longer counting. Why do you ask?
364
365o "nfqs" is the number of calls to force_quiescent_state() since
366 boot.
367
368o "nfqsng" is the number of useless calls to force_quiescent_state(),
369 where there wasn't actually a grace period active. This can
370 happen due to races. The number in parentheses is the difference
371 between "nfqs" and "nfqsng", or the number of times that
372 force_quiescent_state() actually did some real work.
373
374o "fqlh" is the number of calls to force_quiescent_state() that
375 exited immediately (without even being counted in nfqs above)
376 due to contention on ->fqslock.
377
378o Each element of the form "1/1 0:127 ^0" represents one struct
379 rcu_node. Each line represents one level of the hierarchy, from
380 root to leaves. It is best to think of the rcu_data structures
381 as forming yet another level after the leaves. Note that there
382 might be either one, two, or three levels of rcu_node structures,
383 depending on the relationship between CONFIG_RCU_FANOUT and
384 CONFIG_NR_CPUS.
385
386 o The numbers separated by the "/" are the qsmask followed
387 by the qsmaskinit. The qsmask will have one bit
388 set for each entity in the next lower level that
389 has not yet checked in for the current grace period.
390 The qsmaskinit will have one bit for each entity that is
391 currently expected to check in during each grace period.
392 The value of qsmaskinit is assigned to that of qsmask
393 at the beginning of each grace period.
394
395 For example, for "rcu", the qsmask of the first entry
396 of the lowest level is 0x14, meaning that we are still
397 waiting for CPUs 2 and 4 to check in for the current
398 grace period.
399
400 o The numbers separated by the ":" are the range of CPUs
401 served by this struct rcu_node. This can be helpful
402 in working out how the hierarchy is wired together.
403
404 For example, the first entry at the lowest level shows
405 "0:5", indicating that it covers CPUs 0 through 5.
406
407 o The number after the "^" indicates the bit in the
408 next higher level rcu_node structure that this
409 rcu_node structure corresponds to.
410
411 For example, the first entry at the lowest level shows
412 "^0", indicating that it corresponds to bit zero in
413 the first entry at the middle level.