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_TREE_PREEMPT_RCU debugfs Files and Formats
  12
  13 These implementations of RCU provides five debugfs files under the
  14 top-level directory RCU: rcu/rcudata (which displays fields in struct
  15 rcu_data), rcu/rcudata.csv (which is a .csv spreadsheet version of
  16 rcu/rcudata), rcu/rcugp (which displays grace-period counters),
  17 rcu/rcuhier (which displays the struct rcu_node hierarchy), and
  18 rcu/rcu_pending (which displays counts of the reasons that the
  19 rcu_pending() function decided that there was core RCU work to do).
  20
  21 The output of "cat rcu/rcudata" looks as follows:
  22
  23 rcu_sched:
  24   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
  25   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
  26   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
  27   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
  28   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
  29   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
  30   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
  31   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
  32 rcu_bh:
  33   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
  34   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
  35   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
  36   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
  37   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
  38   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
  39   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
  40   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
  41
  42 The first section lists the rcu_data structures for rcu_sched, the second
  43 for rcu_bh.  Note that CONFIG_TREE_PREEMPT_RCU kernels will have an
  44 additional section for rcu_preempt.  Each section has one line per CPU,
  45 or eight for this 8-CPU system.  The fields are as follows:
  46
  47 o       The number at the beginning of each line is the CPU number.
  48         CPUs numbers followed by an exclamation mark are offline,
  49         but have been online at least once since boot.  There will be
  50         no output for CPUs that have never been online, which can be
  51         a good thing in the surprisingly common case where NR_CPUS is
  52         substantially larger than the number of actual CPUs.
  53
  54 o       "c" is the count of grace periods that this CPU believes have
  55         completed.  CPUs in dynticks idle mode may lag quite a ways
  56         behind, for example, CPU 4 under "rcu_sched" above, which has
  57         slept through the past 25 RCU grace periods.  It is not unusual
  58         to see CPUs lagging by thousands of grace periods.
  59
  60 o       "g" is the count of grace periods that this CPU believes have
  61         started.  Again, CPUs in dynticks idle mode may lag behind.
  62         If the "c" and "g" values are equal, this CPU has already
  63         reported a quiescent state for the last RCU grace period that
  64         it is aware of, otherwise, the CPU believes that it owes RCU a
  65         quiescent state.
  66
  67 o       "pq" indicates that this CPU has passed through a quiescent state
  68         for the current grace period.  It is possible for "pq" to be
  69         "1" and "c" different than "g", which indicates that although
  70         the CPU has passed through a quiescent state, either (1) this
  71         CPU has not yet reported that fact, (2) some other CPU has not
  72         yet reported for this grace period, or (3) both.
  73
  74 o       "pqc" indicates which grace period the last-observed quiescent
  75         state for this CPU corresponds to.  This is important for handling
  76         the race between CPU 0 reporting an extended dynticks-idle
  77         quiescent state for CPU 1 and CPU 1 suddenly waking up and
  78         reporting its own quiescent state.  If CPU 1 was the last CPU
  79         for the current grace period, then the CPU that loses this race
  80         will attempt to incorrectly mark CPU 1 as having checked in for
  81         the next grace period!
  82
  83 o       "qp" indicates that RCU still expects a quiescent state from
  84         this CPU.
  85
  86 o       "dt" is the current value of the dyntick counter that is incremented
  87         when entering or leaving dynticks idle state, either by the
  88         scheduler or by irq.  The number after the "/" is the interrupt
  89         nesting depth when in dyntick-idle state, or one greater than
  90         the interrupt-nesting depth otherwise.
  91
  92         This field is displayed only for CONFIG_NO_HZ kernels.
  93
  94 o       "dn" is the current value of the dyntick counter that is incremented
  95         when entering or leaving dynticks idle state via NMI.  If both
  96         the "dt" and "dn" values are even, then this CPU is in dynticks
  97         idle mode and may be ignored by RCU.  If either of these two
  98         counters is odd, then RCU must be alert to the possibility of
  99         an RCU read-side critical section running on this CPU.
 100
 101         This field is displayed only for CONFIG_NO_HZ kernels.
 102
 103 o       "df" is the number of times that some other CPU has forced a
 104         quiescent state on behalf of this CPU due to this CPU being in
 105         dynticks-idle state.
 106
 107         This field is displayed only for CONFIG_NO_HZ kernels.
 108
 109 o       "of" is the number of times that some other CPU has forced a
 110         quiescent state on behalf of this CPU due to this CPU being
 111         offline.  In a perfect world, this might neve happen, but it
 112         turns out that offlining and onlining a CPU can take several grace
 113         periods, and so there is likely to be an extended period of time
 114         when RCU believes that the CPU is online when it really is not.
 115         Please note that erring in the other direction (RCU believing a
 116         CPU is offline when it is really alive and kicking) is a fatal
 117         error, so it makes sense to err conservatively.
 118
 119 o       "ri" is the number of times that RCU has seen fit to send a
 120         reschedule IPI to this CPU in order to get it to report a
 121         quiescent state.
 122
 123 o       "ql" is the number of RCU callbacks currently residing on
 124         this CPU.  This is the total number of callbacks, regardless
 125         of what state they are in (new, waiting for grace period to
 126         start, waiting for grace period to end, ready to invoke).
 127
 128 o       "b" is the batch limit for this CPU.  If more than this number
 129         of RCU callbacks is ready to invoke, then the remainder will
 130         be deferred.
 131
 132 o       "ci" is the number of RCU callbacks that have been invoked for
 133         this CPU.  Note that ci+ql is the number of callbacks that have
 134         been registered in absence of CPU-hotplug activity.
 135
 136 o       "co" is the number of RCU callbacks that have been orphaned due to
 137         this CPU going offline.  These orphaned callbacks have been moved
 138         to an arbitrarily chosen online CPU.
 139
 140 o       "ca" is the number of RCU callbacks that have been adopted due to
 141         other CPUs going offline.  Note that ci+co-ca+ql is the number of
 142         RCU callbacks registered on this CPU.
 143
 144 There is also an rcu/rcudata.csv file with the same information in
 145 comma-separated-variable spreadsheet format.
 146
 147
 148 The output of "cat rcu/rcugp" looks as follows:
 149
 150 rcu_sched: completed=33062  gpnum=33063
 151 rcu_bh: completed=464  gpnum=464
 152
 153 Again, this output is for both "rcu_sched" and "rcu_bh".  Note that
 154 kernels built with CONFIG_TREE_PREEMPT_RCU will have an additional
 155 "rcu_preempt" line.  The fields are taken from the rcu_state structure,
 156 and are as follows:
 157
 158 o       "completed" is the number of grace periods that have completed.
 159         It is comparable to the "c" field from rcu/rcudata in that a
 160         CPU whose "c" field matches the value of "completed" is aware
 161         that the corresponding RCU grace period has completed.
 162
 163 o       "gpnum" is the number of grace periods that have started.  It is
 164         comparable to the "g" field from rcu/rcudata in that a CPU
 165         whose "g" field matches the value of "gpnum" is aware that the
 166         corresponding RCU grace period has started.
 167
 168         If these two fields are equal (as they are for "rcu_bh" above),
 169         then there is no grace period in progress, in other words, RCU
 170         is idle.  On the other hand, if the two fields differ (as they
 171         do for "rcu_sched" above), then an RCU grace period is in progress.
 172
 173
 174 The output of "cat rcu/rcuhier" looks as follows, with very long lines:
 175
 176 c=6902 g=6903 s=2 jfq=3 j=72c7 nfqs=13142/nfqsng=0(13142) fqlh=6
 177 1/1 .>. 0:127 ^0
 178 3/3 .>. 0:35 ^0    0/0 .>. 36:71 ^1    0/0 .>. 72:107 ^2    0/0 .>. 108:127 ^3
 179 3/3f .>. 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
 180 rcu_bh:
 181 c=-226 g=-226 s=1 jfq=-5701 j=72c7 nfqs=88/nfqsng=0(88) fqlh=0
 182 0/1 .>. 0:127 ^0
 183 0/3 .>. 0:35 ^0    0/0 .>. 36:71 ^1    0/0 .>. 72:107 ^2    0/0 .>. 108:127 ^3
 184 0/3f .>. 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
 185
 186 This is once again split into "rcu_sched" and "rcu_bh" portions,
 187 and CONFIG_TREE_PREEMPT_RCU kernels will again have an additional
 188 "rcu_preempt" section.  The fields are as follows:
 189
 190 o       "c" is exactly the same as "completed" under rcu/rcugp.
 191
 192 o       "g" is exactly the same as "gpnum" under rcu/rcugp.
 193
 194 o       "s" is the "signaled" state that drives force_quiescent_state()'s
 195         state machine.
 196
 197 o       "jfq" is the number of jiffies remaining for this grace period
 198         before force_quiescent_state() is invoked to help push things
 199         along.  Note that CPUs in dyntick-idle mode throughout the grace
 200         period will not report on their own, but rather must be check by
 201         some other CPU via force_quiescent_state().
 202
 203 o       "j" is the low-order four hex digits of the jiffies counter.
 204         Yes, Paul did run into a number of problems that turned out to
 205         be due to the jiffies counter no longer counting.  Why do you ask?
 206
 207 o       "nfqs" is the number of calls to force_quiescent_state() since
 208         boot.
 209
 210 o       "nfqsng" is the number of useless calls to force_quiescent_state(),
 211         where there wasn't actually a grace period active.  This can
 212         happen due to races.  The number in parentheses is the difference
 213         between "nfqs" and "nfqsng", or the number of times that
 214         force_quiescent_state() actually did some real work.
 215
 216 o       "fqlh" is the number of calls to force_quiescent_state() that
 217         exited immediately (without even being counted in nfqs above)
 218         due to contention on ->fqslock.
 219
 220 o       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.
 227
 228         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
 237                 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.
 241
 242         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
 251         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.
 265
 266
 267 The output of "cat rcu/rcu_pending" looks as follows:
 268
 269 rcu_sched:
 270   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
 278 rcu_bh:
 279   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
 287
 288 As always, this is once again split into "rcu_sched" and "rcu_bh"
 289 portions, with CONFIG_TREE_PREEMPT_RCU kernels having an additional
 290 "rcu_preempt" section.  The fields are as follows:
 291
 292 o       "np" is the number of times that __rcu_pending() has been invoked
 293         for the corresponding flavor of RCU.
 294
 295 o       "qsp" is the number of times that the RCU was waiting for a
 296         quiescent state from this CPU.
 297
 298 o       "rpq" is the number of times that the CPU had passed through
 299         a quiescent state, but not yet reported it to RCU.
 300
 301 o       "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
 305 o       "cng" is the number of times that this CPU needed another
 306         grace period while RCU was idle.
 307
 308 o       "gpc" is the number of times that an old grace period had
 309         completed, but this CPU was not yet aware of it.
 310
 311 o       "gps" is the number of times that a new grace period had started,
 312         but this CPU was not yet aware of it.
 313
 314 o       "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
 325 o       "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().
 329
 330
 331 CONFIG_TINY_RCU and CONFIG_TINY_PREEMPT_RCU debugfs Files and Formats
 332
 333 These implementations of RCU provides a single debugfs file under the
 334 top-level directory RCU, namely rcu/rcudata, which displays fields in
 335 rcu_bh_ctrlblk, rcu_sched_ctrlblk and, for CONFIG_TINY_PREEMPT_RCU,
 336 rcu_preempt_ctrlblk.
 337
 338 The output of "cat rcu/rcudata" is as follows:
 339
 340 rcu_preempt: qlen=24 gp=1097669 g197/p197/c197 tasks=...
 341              ttb=. btg=no ntb=184 neb=0 nnb=183 j=01f7 bt=0274
 342              normal balk: nt=1097669 gt=0 bt=371 b=0 ny=25073378 nos=0
 343              exp balk: bt=0 nos=0
 344 rcu_sched: qlen: 0
 345 rcu_bh: qlen: 0
 346
 347 This is split into rcu_preempt, rcu_sched, and rcu_bh sections, with the
 348 rcu_preempt section appearing only in CONFIG_TINY_PREEMPT_RCU builds.
 349 The last three lines of the rcu_preempt section appear only in
 350 CONFIG_RCU_BOOST kernel builds.  The fields are as follows:
 351
 352 o       "qlen" is the number of RCU callbacks currently waiting either
 353         for an RCU grace period or waiting to be invoked.  This is the
 354         only field present for rcu_sched and rcu_bh, due to the
 355         short-circuiting of grace period in those two cases.
 356
 357 o       "gp" is the number of grace periods that have completed.
 358
 359 o       "g197/p197/c197" displays the grace-period state, with the
 360         "g" number being the number of grace periods that have started
 361         (mod 256), the "p" number being the number of grace periods
 362         that the CPU has responded to (also mod 256), and the "c"
 363         number being the number of grace periods that have completed
 364         (once again mode 256).
 365
 366         Why have both "gp" and "g"?  Because the data flowing into
 367         "gp" is only present in a CONFIG_RCU_TRACE kernel.
 368
 369 o       "tasks" is a set of bits.  The first bit is "T" if there are
 370         currently tasks that have recently blocked within an RCU
 371         read-side critical section, the second bit is "N" if any of the
 372         aforementioned tasks are blocking the current RCU grace period,
 373         and the third bit is "E" if any of the aforementioned tasks are
 374         blocking the current expedited grace period.  Each bit is "."
 375         if the corresponding condition does not hold.
 376
 377 o       "ttb" is a single bit.  It is "B" if any of the blocked tasks
 378         need to be priority boosted and "." otherwise.
 379
 380 o       "btg" indicates whether boosting has been carried out during
 381         the current grace period, with "exp" indicating that boosting
 382         is in progress for an expedited grace period, "no" indicating
 383         that boosting has not yet started for a normal grace period,
 384         "begun" indicating that boosting has bebug for a normal grace
 385         period, and "done" indicating that boosting has completed for
 386         a normal grace period.
 387
 388 o       "ntb" is the total number of tasks subjected to RCU priority boosting
 389         periods since boot.
 390
 391 o       "neb" is the number of expedited grace periods that have had
 392         to resort to RCU priority boosting since boot.
 393
 394 o       "nnb" is the number of normal grace periods that have had
 395         to resort to RCU priority boosting since boot.
 396
 397 o       "j" is the low-order 12 bits of the jiffies counter in hexadecimal.
 398
 399 o       "bt" is the low-order 12 bits of the value that the jiffies counter
 400         will have at the next time that boosting is scheduled to begin.
 401
 402 o       In the line beginning with "normal balk", the fields are as follows:
 403
 404         o       "nt" is the number of times that the system balked from
 405                 boosting because there were no blocked tasks to boost.
 406                 Note that the system will balk from boosting even if the
 407                 grace period is overdue when the currently running task
 408                 is looping within an RCU read-side critical section.
 409                 There is no point in boosting in this case, because
 410                 boosting a running task won't make it run any faster.
 411
 412         o       "gt" is the number of times that the system balked
 413                 from boosting because, although there were blocked tasks,
 414                 none of them were preventing the current grace period
 415                 from completing.
 416
 417         o       "bt" is the number of times that the system balked
 418                 from boosting because boosting was already in progress.
 419
 420         o       "b" is the number of times that the system balked from
 421                 boosting because boosting had already completed for
 422                 the grace period in question.
 423
 424         o       "ny" is the number of times that the system balked from
 425                 boosting because it was not yet time to start boosting
 426                 the grace period in question.
 427
 428         o       "nos" is the number of times that the system balked from
 429                 boosting for inexplicable ("not otherwise specified")
 430                 reasons.  This can actually happen due to races involving
 431                 increments of the jiffies counter.
 432
 433 o       In the line beginning with "exp balk", the fields are as follows:
 434
 435         o       "bt" is the number of times that the system balked from
 436                 boosting because there were no blocked tasks to boost.
 437
 438         o       "nos" is the number of times that the system balked from
 439                  boosting for inexplicable ("not otherwise specified")
 440                  reasons.