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Paul E. McKenney64db4cf2008-12-18 21:55:32 +01001CONFIG_RCU_TRACE debugfs Files and Formats
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3
Paul E. McKenneybd58b432009-10-14 10:15:54 -07004The rcutree implementation of RCU provides debugfs trace output that
5summarizes counters and state. This information is useful for debugging
6RCU itself, and can sometimes also help to debug abuses of RCU.
7The following sections describe the debugfs files and formats.
Paul E. McKenney64db4cf2008-12-18 21:55:32 +01008
9
10Hierarchical RCU debugfs Files and Formats
11
12This implementation of RCU provides three debugfs files under the
13top-level directory RCU: rcu/rcudata (which displays fields in struct
14rcu_data), rcu/rcugp (which displays grace-period counters), and
15rcu/rcuhier (which displays the struct rcu_node hierarchy).
16
17The output of "cat rcu/rcudata" looks as follows:
18
Paul E. McKenneyd6714c22009-08-22 13:56:46 -070019rcu_sched:
Paul E. McKenney6fd9b3a2009-04-13 21:31:18 -070020 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
21 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
22 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
23 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
24 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
25 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
26 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
27 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
Paul E. McKenney64db4cf2008-12-18 21:55:32 +010028rcu_bh:
Paul E. McKenney6fd9b3a2009-04-13 21:31:18 -070029 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
30 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
31 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
32 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
33 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
34 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
35 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
36 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
Paul E. McKenney64db4cf2008-12-18 21:55:32 +010037
Paul E. McKenneybd58b432009-10-14 10:15:54 -070038The first section lists the rcu_data structures for rcu_sched, the second
39for rcu_bh. Note that CONFIG_TREE_PREEMPT_RCU kernels will have an
40additional section for rcu_preempt. Each section has one line per CPU,
41or eight for this 8-CPU system. The fields are as follows:
Paul E. McKenney64db4cf2008-12-18 21:55:32 +010042
43o The number at the beginning of each line is the CPU number.
44 CPUs numbers followed by an exclamation mark are offline,
45 but have been online at least once since boot. There will be
46 no output for CPUs that have never been online, which can be
47 a good thing in the surprisingly common case where NR_CPUS is
48 substantially larger than the number of actual CPUs.
49
50o "c" is the count of grace periods that this CPU believes have
51 completed. CPUs in dynticks idle mode may lag quite a ways
Paul E. McKenneybd58b432009-10-14 10:15:54 -070052 behind, for example, CPU 4 under "rcu_sched" above, which has
53 slept through the past 25 RCU grace periods. It is not unusual
54 to see CPUs lagging by thousands of grace periods.
Paul E. McKenney64db4cf2008-12-18 21:55:32 +010055
56o "g" is the count of grace periods that this CPU believes have
57 started. Again, CPUs in dynticks idle mode may lag behind.
58 If the "c" and "g" values are equal, this CPU has already
59 reported a quiescent state for the last RCU grace period that
60 it is aware of, otherwise, the CPU believes that it owes RCU a
61 quiescent state.
62
63o "pq" indicates that this CPU has passed through a quiescent state
64 for the current grace period. It is possible for "pq" to be
65 "1" and "c" different than "g", which indicates that although
66 the CPU has passed through a quiescent state, either (1) this
67 CPU has not yet reported that fact, (2) some other CPU has not
68 yet reported for this grace period, or (3) both.
69
70o "pqc" indicates which grace period the last-observed quiescent
71 state for this CPU corresponds to. This is important for handling
72 the race between CPU 0 reporting an extended dynticks-idle
73 quiescent state for CPU 1 and CPU 1 suddenly waking up and
74 reporting its own quiescent state. If CPU 1 was the last CPU
75 for the current grace period, then the CPU that loses this race
76 will attempt to incorrectly mark CPU 1 as having checked in for
77 the next grace period!
78
79o "qp" indicates that RCU still expects a quiescent state from
80 this CPU.
81
Paul E. McKenney64db4cf2008-12-18 21:55:32 +010082o "dt" is the current value of the dyntick counter that is incremented
83 when entering or leaving dynticks idle state, either by the
84 scheduler or by irq. The number after the "/" is the interrupt
85 nesting depth when in dyntick-idle state, or one greater than
86 the interrupt-nesting depth otherwise.
87
88 This field is displayed only for CONFIG_NO_HZ kernels.
89
90o "dn" is the current value of the dyntick counter that is incremented
91 when entering or leaving dynticks idle state via NMI. If both
92 the "dt" and "dn" values are even, then this CPU is in dynticks
93 idle mode and may be ignored by RCU. If either of these two
94 counters is odd, then RCU must be alert to the possibility of
95 an RCU read-side critical section running on this CPU.
96
97 This field is displayed only for CONFIG_NO_HZ kernels.
98
99o "df" is the number of times that some other CPU has forced a
100 quiescent state on behalf of this CPU due to this CPU being in
101 dynticks-idle state.
102
103 This field is displayed only for CONFIG_NO_HZ kernels.
104
105o "of" is the number of times that some other CPU has forced a
106 quiescent state on behalf of this CPU due to this CPU being
107 offline. In a perfect world, this might neve happen, but it
108 turns out that offlining and onlining a CPU can take several grace
109 periods, and so there is likely to be an extended period of time
110 when RCU believes that the CPU is online when it really is not.
111 Please note that erring in the other direction (RCU believing a
112 CPU is offline when it is really alive and kicking) is a fatal
113 error, so it makes sense to err conservatively.
114
115o "ri" is the number of times that RCU has seen fit to send a
116 reschedule IPI to this CPU in order to get it to report a
117 quiescent state.
118
119o "ql" is the number of RCU callbacks currently residing on
120 this CPU. This is the total number of callbacks, regardless
121 of what state they are in (new, waiting for grace period to
122 start, waiting for grace period to end, ready to invoke).
123
124o "b" is the batch limit for this CPU. If more than this number
125 of RCU callbacks is ready to invoke, then the remainder will
126 be deferred.
127
Paul E. McKenney269dcc12010-09-07 14:23:09 -0700128o "ci" is the number of RCU callbacks that have been invoked for
129 this CPU. Note that ci+ql is the number of callbacks that have
130 been registered in absence of CPU-hotplug activity.
131
132o "co" is the number of RCU callbacks that have been orphaned due to
133 this CPU going offline.
134
135o "ca" is the number of RCU callbacks that have been adopted due to
136 other CPUs going offline. Note that ci+co-ca+ql is the number of
137 RCU callbacks registered on this CPU.
138
Paul E. McKenney6fd9b3a2009-04-13 21:31:18 -0700139There is also an rcu/rcudata.csv file with the same information in
140comma-separated-variable spreadsheet format.
141
Paul E. McKenney64db4cf2008-12-18 21:55:32 +0100142
143The output of "cat rcu/rcugp" looks as follows:
144
Paul E. McKenneyd6714c22009-08-22 13:56:46 -0700145rcu_sched: completed=33062 gpnum=33063
Paul E. McKenney64db4cf2008-12-18 21:55:32 +0100146rcu_bh: completed=464 gpnum=464
147
Paul E. McKenneybd58b432009-10-14 10:15:54 -0700148Again, this output is for both "rcu_sched" and "rcu_bh". Note that
149kernels built with CONFIG_TREE_PREEMPT_RCU will have an additional
150"rcu_preempt" line. The fields are taken from the rcu_state structure,
151and are as follows:
Paul E. McKenney64db4cf2008-12-18 21:55:32 +0100152
153o "completed" is the number of grace periods that have completed.
154 It is comparable to the "c" field from rcu/rcudata in that a
155 CPU whose "c" field matches the value of "completed" is aware
156 that the corresponding RCU grace period has completed.
157
158o "gpnum" is the number of grace periods that have started. It is
159 comparable to the "g" field from rcu/rcudata in that a CPU
160 whose "g" field matches the value of "gpnum" is aware that the
161 corresponding RCU grace period has started.
162
163 If these two fields are equal (as they are for "rcu_bh" above),
164 then there is no grace period in progress, in other words, RCU
165 is idle. On the other hand, if the two fields differ (as they
Paul E. McKenneybd58b432009-10-14 10:15:54 -0700166 do for "rcu_sched" above), then an RCU grace period is in progress.
Paul E. McKenney64db4cf2008-12-18 21:55:32 +0100167
168
169The output of "cat rcu/rcuhier" looks as follows, with very long lines:
170
Paul E. McKenneybd58b432009-10-14 10:15:54 -0700171c=6902 g=6903 s=2 jfq=3 j=72c7 nfqs=13142/nfqsng=0(13142) fqlh=6 oqlen=0
Paul E. McKenney0edf1a62009-10-14 10:15:59 -07001721/1 .>. 0:127 ^0
1733/3 .>. 0:35 ^0 0/0 .>. 36:71 ^1 0/0 .>. 72:107 ^2 0/0 .>. 108:127 ^3
1743/3f .>. 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
Paul E. McKenney64db4cf2008-12-18 21:55:32 +0100175rcu_bh:
Paul E. McKenneybd58b432009-10-14 10:15:54 -0700176c=-226 g=-226 s=1 jfq=-5701 j=72c7 nfqs=88/nfqsng=0(88) fqlh=0 oqlen=0
Paul E. McKenney0edf1a62009-10-14 10:15:59 -07001770/1 .>. 0:127 ^0
1780/3 .>. 0:35 ^0 0/0 .>. 36:71 ^1 0/0 .>. 72:107 ^2 0/0 .>. 108:127 ^3
1790/3f .>. 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
Paul E. McKenney64db4cf2008-12-18 21:55:32 +0100180
Paul E. McKenneybd58b432009-10-14 10:15:54 -0700181This is once again split into "rcu_sched" and "rcu_bh" portions,
182and CONFIG_TREE_PREEMPT_RCU kernels will again have an additional
183"rcu_preempt" section. The fields are as follows:
Paul E. McKenney64db4cf2008-12-18 21:55:32 +0100184
185o "c" is exactly the same as "completed" under rcu/rcugp.
186
187o "g" is exactly the same as "gpnum" under rcu/rcugp.
188
189o "s" is the "signaled" state that drives force_quiescent_state()'s
190 state machine.
191
192o "jfq" is the number of jiffies remaining for this grace period
193 before force_quiescent_state() is invoked to help push things
Paul E. McKenney269dcc12010-09-07 14:23:09 -0700194 along. Note that CPUs in dyntick-idle mode throughout the grace
Paul E. McKenney64db4cf2008-12-18 21:55:32 +0100195 period will not report on their own, but rather must be check by
196 some other CPU via force_quiescent_state().
197
198o "j" is the low-order four hex digits of the jiffies counter.
199 Yes, Paul did run into a number of problems that turned out to
200 be due to the jiffies counter no longer counting. Why do you ask?
201
202o "nfqs" is the number of calls to force_quiescent_state() since
203 boot.
204
205o "nfqsng" is the number of useless calls to force_quiescent_state(),
206 where there wasn't actually a grace period active. This can
207 happen due to races. The number in parentheses is the difference
208 between "nfqs" and "nfqsng", or the number of times that
209 force_quiescent_state() actually did some real work.
210
211o "fqlh" is the number of calls to force_quiescent_state() that
212 exited immediately (without even being counted in nfqs above)
213 due to contention on ->fqslock.
214
Paul E. McKenneybd58b432009-10-14 10:15:54 -0700215o "oqlen" is the number of callbacks on the "orphan" callback
216 list. RCU callbacks are placed on this list by CPUs going
217 offline, and are "adopted" either by the CPU helping the outgoing
218 CPU or by the next rcu_barrier*() call, whichever comes first.
219
Paul E. McKenney64db4cf2008-12-18 21:55:32 +0100220o Each element of the form "1/1 0:127 ^0" represents one struct
221 rcu_node. Each line represents one level of the hierarchy, from
222 root to leaves. It is best to think of the rcu_data structures
223 as forming yet another level after the leaves. Note that there
224 might be either one, two, or three levels of rcu_node structures,
225 depending on the relationship between CONFIG_RCU_FANOUT and
226 CONFIG_NR_CPUS.
Paul E. McKenney0edf1a62009-10-14 10:15:59 -0700227
Paul E. McKenney64db4cf2008-12-18 21:55:32 +0100228 o The numbers separated by the "/" are the qsmask followed
229 by the qsmaskinit. The qsmask will have one bit
230 set for each entity in the next lower level that
231 has not yet checked in for the current grace period.
232 The qsmaskinit will have one bit for each entity that is
233 currently expected to check in during each grace period.
234 The value of qsmaskinit is assigned to that of qsmask
235 at the beginning of each grace period.
236
Paul E. McKenneybd58b432009-10-14 10:15:54 -0700237 For example, for "rcu_sched", the qsmask of the first
238 entry of the lowest level is 0x14, meaning that we
239 are still waiting for CPUs 2 and 4 to check in for the
240 current grace period.
Paul E. McKenney64db4cf2008-12-18 21:55:32 +0100241
Paul E. McKenney0edf1a62009-10-14 10:15:59 -0700242 o The characters separated by the ">" indicate the state
243 of the blocked-tasks lists. A "T" preceding the ">"
244 indicates that at least one task blocked in an RCU
245 read-side critical section blocks the current grace
246 period, while a "." preceding the ">" indicates otherwise.
247 The character following the ">" indicates similarly for
248 the next grace period. A "T" should appear in this
249 field only for rcu-preempt.
250
Paul E. McKenney64db4cf2008-12-18 21:55:32 +0100251 o The numbers separated by the ":" are the range of CPUs
252 served by this struct rcu_node. This can be helpful
253 in working out how the hierarchy is wired together.
254
255 For example, the first entry at the lowest level shows
256 "0:5", indicating that it covers CPUs 0 through 5.
257
258 o The number after the "^" indicates the bit in the
259 next higher level rcu_node structure that this
260 rcu_node structure corresponds to.
261
262 For example, the first entry at the lowest level shows
263 "^0", indicating that it corresponds to bit zero in
264 the first entry at the middle level.
Paul E. McKenney6fd9b3a2009-04-13 21:31:18 -0700265
266
267The output of "cat rcu/rcu_pending" looks as follows:
268
Paul E. McKenneyd6714c22009-08-22 13:56:46 -0700269rcu_sched:
Paul E. McKenneyd21670a2010-04-14 17:39:26 -0700270 0 np=255892 qsp=53936 rpq=85 cbr=0 cng=14417 gpc=10033 gps=24320 nf=6445 nn=146741
271 1 np=261224 qsp=54638 rpq=33 cbr=0 cng=25723 gpc=16310 gps=2849 nf=5912 nn=155792
272 2 np=237496 qsp=49664 rpq=23 cbr=0 cng=2762 gpc=45478 gps=1762 nf=1201 nn=136629
273 3 np=236249 qsp=48766 rpq=98 cbr=0 cng=286 gpc=48049 gps=1218 nf=207 nn=137723
274 4 np=221310 qsp=46850 rpq=7 cbr=0 cng=26 gpc=43161 gps=4634 nf=3529 nn=123110
275 5 np=237332 qsp=48449 rpq=9 cbr=0 cng=54 gpc=47920 gps=3252 nf=201 nn=137456
276 6 np=219995 qsp=46718 rpq=12 cbr=0 cng=50 gpc=42098 gps=6093 nf=4202 nn=120834
277 7 np=249893 qsp=49390 rpq=42 cbr=0 cng=72 gpc=38400 gps=17102 nf=41 nn=144888
Paul E. McKenney6fd9b3a2009-04-13 21:31:18 -0700278rcu_bh:
Paul E. McKenneyd21670a2010-04-14 17:39:26 -0700279 0 np=146741 qsp=1419 rpq=6 cbr=0 cng=6 gpc=0 gps=0 nf=2 nn=145314
280 1 np=155792 qsp=12597 rpq=3 cbr=0 cng=0 gpc=4 gps=8 nf=3 nn=143180
281 2 np=136629 qsp=18680 rpq=1 cbr=0 cng=0 gpc=7 gps=6 nf=0 nn=117936
282 3 np=137723 qsp=2843 rpq=0 cbr=0 cng=0 gpc=10 gps=7 nf=0 nn=134863
283 4 np=123110 qsp=12433 rpq=0 cbr=0 cng=0 gpc=4 gps=2 nf=0 nn=110671
284 5 np=137456 qsp=4210 rpq=1 cbr=0 cng=0 gpc=6 gps=5 nf=0 nn=133235
285 6 np=120834 qsp=9902 rpq=2 cbr=0 cng=0 gpc=6 gps=3 nf=2 nn=110921
286 7 np=144888 qsp=26336 rpq=0 cbr=0 cng=0 gpc=8 gps=2 nf=0 nn=118542
Paul E. McKenney6fd9b3a2009-04-13 21:31:18 -0700287
Paul E. McKenneybd58b432009-10-14 10:15:54 -0700288As always, this is once again split into "rcu_sched" and "rcu_bh"
289portions, with CONFIG_TREE_PREEMPT_RCU kernels having an additional
290"rcu_preempt" section. The fields are as follows:
Paul E. McKenney6fd9b3a2009-04-13 21:31:18 -0700291
292o "np" is the number of times that __rcu_pending() has been invoked
293 for the corresponding flavor of RCU.
294
295o "qsp" is the number of times that the RCU was waiting for a
296 quiescent state from this CPU.
297
Paul E. McKenneyd21670a2010-04-14 17:39:26 -0700298o "rpq" is the number of times that the CPU had passed through
299 a quiescent state, but not yet reported it to RCU.
300
Paul E. McKenney6fd9b3a2009-04-13 21:31:18 -0700301o "cbr" is the number of times that this CPU had RCU callbacks
302 that had passed through a grace period, and were thus ready
303 to be invoked.
304
305o "cng" is the number of times that this CPU needed another
306 grace period while RCU was idle.
307
308o "gpc" is the number of times that an old grace period had
309 completed, but this CPU was not yet aware of it.
310
311o "gps" is the number of times that a new grace period had started,
312 but this CPU was not yet aware of it.
313
314o "nf" is the number of times that this CPU suspected that the
315 current grace period had run for too long, and thus needed to
316 be forced.
317
318 Please note that "forcing" consists of sending resched IPIs
319 to holdout CPUs. If that CPU really still is in an old RCU
320 read-side critical section, then we really do have to wait for it.
321 The assumption behing "forcing" is that the CPU is not still in
322 an old RCU read-side critical section, but has not yet responded
323 for some other reason.
324
325o "nn" is the number of times that this CPU needed nothing. Alert
326 readers will note that the rcu "nn" number for a given CPU very
327 closely matches the rcu_bh "np" number for that same CPU. This
328 is due to short-circuit evaluation in rcu_pending().