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 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 d=21872 w=0 tf=0 wd1=0 wd2=0 n=0 sc=21872 dt=21872 dl=0 dx=21872
 241
 242 These fields are as follows:
 243
 244 o       "s" is the starting sequence number.
 245
 246 o       "d" is the ending sequence number.  When the starting and ending
 247         numbers differ, there is an expedited grace period in progress.
 248
 249 o       "w" is the number of times that the sequence numbers have been
 250         in danger of wrapping.
 251
 252 o       "tf" is the number of times that contention has resulted in a
 253         failure to begin an expedited grace period.
 254
 255 o       "wd1" and "wd2" are the number of times that an attempt to
 256         start an expedited grace period found that someone else had
 257         completed an expedited grace period that satisfies the
 258         attempted request.  "Our work is done."
 259
 260 o       "n" is number of times that contention was so great that
 261         the request was demoted from an expedited grace period to
 262         a normal grace period.
 263
 264 o       "sc" is the number of times that the attempt to start a
 265         new expedited grace period succeeded.
 266
 267 o       "dt" is the number of times that we attempted to update
 268         the "d" counter.
 269
 270 o       "dl" is the number of times that we failed to update the "d"
 271         counter.
 272
 273 o       "dx" is the number of times that we succeeded in updating
 274         the "d" counter.
 275
 276
 277 The output of "cat rcu/rcu_preempt/rcugp" looks as follows:
 278
 279 completed=31249  gpnum=31250  age=1  max=18
 280
 281 These fields are taken from the rcu_state structure, and are as follows:
 282
 283 o       "completed" is the number of grace periods that have completed.
 284         It is comparable to the "c" field from rcu/rcudata in that a
 285         CPU whose "c" field matches the value of "completed" is aware
 286         that the corresponding RCU grace period has completed.
 287
 288 o       "gpnum" is the number of grace periods that have started.  It is
 289         similarly comparable to the "g" field from rcu/rcudata in that
 290         a CPU whose "g" field matches the value of "gpnum" is aware that
 291         the corresponding RCU grace period has started.
 292
 293         If these two fields are equal, then there is no grace period
 294         in progress, in other words, RCU is idle.  On the other hand,
 295         if the two fields differ (as they are above), then an RCU grace
 296         period is in progress.
 297
 298 o       "age" is the number of jiffies that the current grace period
 299         has extended for, or zero if there is no grace period currently
 300         in effect.
 301
 302 o       "max" is the age in jiffies of the longest-duration grace period
 303         thus far.
 304
 305 The output of "cat rcu/rcu_preempt/rcuhier" looks as follows:
 306
 307 c=14407 g=14408 s=0 jfq=2 j=c863 nfqs=12040/nfqsng=0(12040) fqlh=1051 oqlen=0/0
 308 3/3 ..>. 0:7 ^0
 309 e/e ..>. 0:3 ^0    d/d ..>. 4:7 ^1
 310
 311 The fields are as follows:
 312
 313 o       "c" is exactly the same as "completed" under rcu/rcu_preempt/rcugp.
 314
 315 o       "g" is exactly the same as "gpnum" under rcu/rcu_preempt/rcugp.
 316
 317 o       "s" is the current state of the force_quiescent_state()
 318         state machine.
 319
 320 o       "jfq" is the number of jiffies remaining for this grace period
 321         before force_quiescent_state() is invoked to help push things
 322         along.  Note that CPUs in idle mode throughout the grace period
 323         will not report on their own, but rather must be check by some
 324         other CPU via force_quiescent_state().
 325
 326 o       "j" is the low-order four hex digits of the jiffies counter.
 327         Yes, Paul did run into a number of problems that turned out to
 328         be due to the jiffies counter no longer counting.  Why do you ask?
 329
 330 o       "nfqs" is the number of calls to force_quiescent_state() since
 331         boot.
 332
 333 o       "nfqsng" is the number of useless calls to force_quiescent_state(),
 334         where there wasn't actually a grace period active.  This can
 335         no longer happen due to grace-period processing being pushed
 336         into a kthread.  The number in parentheses is the difference
 337         between "nfqs" and "nfqsng", or the number of times that
 338         force_quiescent_state() actually did some real work.
 339
 340 o       "fqlh" is the number of calls to force_quiescent_state() that
 341         exited immediately (without even being counted in nfqs above)
 342         due to contention on ->fqslock.
 343
 344 o       Each element of the form "3/3 ..>. 0:7 ^0" represents one rcu_node
 345         structure.  Each line represents one level of the hierarchy,
 346         from root to leaves.  It is best to think of the rcu_data
 347         structures as forming yet another level after the leaves.
 348         Note that there might be either one, two, three, or even four
 349         levels of rcu_node structures, depending on the relationship
 350         between CONFIG_RCU_FANOUT, CONFIG_RCU_FANOUT_LEAF (possibly
 351         adjusted using the rcu_fanout_leaf kernel boot parameter), and
 352         CONFIG_NR_CPUS (possibly adjusted using the nr_cpu_ids count of
 353         possible CPUs for the booting hardware).
 354
 355         o       The numbers separated by the "/" are the qsmask followed
 356                 by the qsmaskinit.  The qsmask will have one bit
 357                 set for each entity in the next lower level that has
 358                 not yet checked in for the current grace period ("e"
 359                 indicating CPUs 5, 6, and 7 in the example above).
 360                 The qsmaskinit will have one bit for each entity that is
 361                 currently expected to check in during each grace period.
 362                 The value of qsmaskinit is assigned to that of qsmask
 363                 at the beginning of each grace period.
 364
 365         o       The characters separated by the ">" indicate the state
 366                 of the blocked-tasks lists.  A "G" preceding the ">"
 367                 indicates that at least one task blocked in an RCU
 368                 read-side critical section blocks the current grace
 369                 period, while a "E" preceding the ">" indicates that
 370                 at least one task blocked in an RCU read-side critical
 371                 section blocks the current expedited grace period.
 372                 A "T" character following the ">" indicates that at
 373                 least one task is blocked within an RCU read-side
 374                 critical section, regardless of whether any current
 375                 grace period (expedited or normal) is inconvenienced.
 376                 A "." character appears if the corresponding condition
 377                 does not hold, so that "..>." indicates that no tasks
 378                 are blocked.  In contrast, "GE>T" indicates maximal
 379                 inconvenience from blocked tasks.  CONFIG_TREE_RCU
 380                 builds of the kernel will always show "..>.".
 381
 382         o       The numbers separated by the ":" are the range of CPUs
 383                 served by this struct rcu_node.  This can be helpful
 384                 in working out how the hierarchy is wired together.
 385
 386                 For example, the example rcu_node structure shown above
 387                 has "0:7", indicating that it covers CPUs 0 through 7.
 388
 389         o       The number after the "^" indicates the bit in the
 390                 next higher level rcu_node structure that this rcu_node
 391                 structure corresponds to.  For example, the "d/d ..>. 4:7
 392                 ^1" has a "1" in this position, indicating that it
 393                 corresponds to the "1" bit in the "3" shown in the
 394                 "3/3 ..>. 0:7 ^0" entry on the next level up.
 395
 396
 397 The output of "cat rcu/rcu_sched/rcu_pending" looks as follows:
 398
 399   0!np=26111 qsp=29 rpq=5386 cbr=1 cng=570 gpc=3674 gps=577 nn=15903 ndw=0
 400   1!np=28913 qsp=35 rpq=6097 cbr=1 cng=448 gpc=3700 gps=554 nn=18113 ndw=0
 401   2!np=32740 qsp=37 rpq=6202 cbr=0 cng=476 gpc=4627 gps=546 nn=20889 ndw=0
 402   3 np=23679 qsp=22 rpq=5044 cbr=1 cng=415 gpc=3403 gps=347 nn=14469 ndw=0
 403   4!np=30714 qsp=4 rpq=5574 cbr=0 cng=528 gpc=3931 gps=639 nn=20042 ndw=0
 404   5 np=28910 qsp=2 rpq=5246 cbr=0 cng=428 gpc=4105 gps=709 nn=18422 ndw=0
 405   6!np=38648 qsp=5 rpq=7076 cbr=0 cng=840 gpc=4072 gps=961 nn=25699 ndw=0
 406   7 np=37275 qsp=2 rpq=6873 cbr=0 cng=868 gpc=3416 gps=971 nn=25147 ndw=0
 407
 408 The fields are as follows:
 409
 410 o       The leading number is the CPU number, with "!" indicating
 411         an offline CPU.
 412
 413 o       "np" is the number of times that __rcu_pending() has been invoked
 414         for the corresponding flavor of RCU.
 415
 416 o       "qsp" is the number of times that the RCU was waiting for a
 417         quiescent state from this CPU.
 418
 419 o       "rpq" is the number of times that the CPU had passed through
 420         a quiescent state, but not yet reported it to RCU.
 421
 422 o       "cbr" is the number of times that this CPU had RCU callbacks
 423         that had passed through a grace period, and were thus ready
 424         to be invoked.
 425
 426 o       "cng" is the number of times that this CPU needed another
 427         grace period while RCU was idle.
 428
 429 o       "gpc" is the number of times that an old grace period had
 430         completed, but this CPU was not yet aware of it.
 431
 432 o       "gps" is the number of times that a new grace period had started,
 433         but this CPU was not yet aware of it.
 434
 435 o       "ndw" is the number of times that a wakeup of an rcuo
 436         callback-offload kthread had to be deferred in order to avoid
 437         deadlock.
 438
 439 o       "nn" is the number of times that this CPU needed nothing.
 440
 441
 442 The output of "cat rcu/rcuboost" looks as follows:
 443
 444 0:3 tasks=.... kt=W ntb=0 neb=0 nnb=0 j=c864 bt=c894
 445     balk: nt=0 egt=4695 bt=0 nb=0 ny=56 nos=0
 446 4:7 tasks=.... kt=W ntb=0 neb=0 nnb=0 j=c864 bt=c894
 447     balk: nt=0 egt=6541 bt=0 nb=0 ny=126 nos=0
 448
 449 This information is output only for rcu_preempt.  Each two-line entry
 450 corresponds to a leaf rcu_node structure.  The fields are as follows:
 451
 452 o       "n:m" is the CPU-number range for the corresponding two-line
 453         entry.  In the sample output above, the first entry covers
 454         CPUs zero through three and the second entry covers CPUs four
 455         through seven.
 456
 457 o       "tasks=TNEB" gives the state of the various segments of the
 458         rnp->blocked_tasks list:
 459
 460         "T"     This indicates that there are some tasks that blocked
 461                 while running on one of the corresponding CPUs while
 462                 in an RCU read-side critical section.
 463
 464         "N"     This indicates that some of the blocked tasks are preventing
 465                 the current normal (non-expedited) grace period from
 466                 completing.
 467
 468         "E"     This indicates that some of the blocked tasks are preventing
 469                 the current expedited grace period from completing.
 470
 471         "B"     This indicates that some of the blocked tasks are in
 472                 need of RCU priority boosting.
 473
 474         Each character is replaced with "." if the corresponding
 475         condition does not hold.
 476
 477 o       "kt" is the state of the RCU priority-boosting kernel
 478         thread associated with the corresponding rcu_node structure.
 479         The state can be one of the following:
 480
 481         "S"     The kernel thread is stopped, in other words, all
 482                 CPUs corresponding to this rcu_node structure are
 483                 offline.
 484
 485         "R"     The kernel thread is running.
 486
 487         "W"     The kernel thread is waiting because there is no work
 488                 for it to do.
 489
 490         "Y"     The kernel thread is yielding to avoid hogging CPU.
 491
 492         "?"     Unknown value, indicates a bug.
 493
 494 o       "ntb" is the number of tasks boosted.
 495
 496 o       "neb" is the number of tasks boosted in order to complete an
 497         expedited grace period.
 498
 499 o       "nnb" is the number of tasks boosted in order to complete a
 500         normal (non-expedited) grace period.  When boosting a task
 501         that was blocking both an expedited and a normal grace period,
 502         it is counted against the expedited total above.
 503
 504 o       "j" is the low-order 16 bits of the jiffies counter in
 505         hexadecimal.
 506
 507 o       "bt" is the low-order 16 bits of the value that the jiffies
 508         counter will have when we next start boosting, assuming that
 509         the current grace period does not end beforehand.  This is
 510         also in hexadecimal.
 511
 512 o       "balk: nt" counts the number of times we didn't boost (in
 513         other words, we balked) even though it was time to boost because
 514         there were no blocked tasks to boost.  This situation occurs
 515         when there is one blocked task on one rcu_node structure and
 516         none on some other rcu_node structure.
 517
 518 o       "egt" counts the number of times we balked because although
 519         there were blocked tasks, none of them were blocking the
 520         current grace period, whether expedited or otherwise.
 521
 522 o       "bt" counts the number of times we balked because boosting
 523         had already been initiated for the current grace period.
 524
 525 o       "nb" counts the number of times we balked because there
 526         was at least one task blocking the current non-expedited grace
 527         period that never had blocked.  If it is already running, it
 528         just won't help to boost its priority!
 529
 530 o       "ny" counts the number of times we balked because it was
 531         not yet time to start boosting.
 532
 533 o       "nos" counts the number of times we balked for other
 534         reasons, e.g., the grace period ended first.
 535
 536
 537 CONFIG_TINY_RCU debugfs Files and Formats
 538
 539 These implementations of RCU provides a single debugfs file under the
 540 top-level directory RCU, namely rcu/rcudata, which displays fields in
 541 rcu_bh_ctrlblk and rcu_sched_ctrlblk.
 542
 543 The output of "cat rcu/rcudata" is as follows:
 544
 545 rcu_sched: qlen: 0
 546 rcu_bh: qlen: 0
 547
 548 This is split into rcu_sched and rcu_bh sections.  The field is as
 549 follows:
 550
 551 o       "qlen" is the number of RCU callbacks currently waiting either
 552         for an RCU grace period or waiting to be invoked.  This is the
 553         only field present for rcu_sched and rcu_bh, due to the
 554         short-circuiting of grace period in those two cases.