Documentation/RCU/trace.txt

   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 cnq=1/0:1 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 cnq=1/0:0 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 cnq=1/1:1 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 cnq=1/1:0 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 cnq=1/0:1 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 cnq=1/0:1 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 cnq=1/0:1 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 cnq=1/0:0 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 cnq=1/0:1 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 cnq=1/0:0 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 cnq=1/0:0 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 cnq=1/0:0 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 cnq=1/0:1 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 cnq=1/0:0 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 cnq=1/0:0 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 cnq=1/0:1 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.