Documentation/RCU/trace.txt

   1 CONFIG_RCU_TRACE debugfs Files and Formats
   2
   3
   4 The rcupreempt and rcutree 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 Note that the rcuclassic implementation of RCU does not provide debugfs
   8 trace output.
   9
  10 The following sections describe the debugfs files and formats for
  11 preemptable RCU (rcupreempt) and hierarchical RCU (rcutree).
  12
  13
  14 Preemptable RCU debugfs Files and Formats
  15
  16 This implementation of RCU provides three debugfs files under the
  17 top-level directory RCU: rcu/rcuctrs (which displays the per-CPU
  18 counters used by preemptable RCU) rcu/rcugp (which displays grace-period
  19 counters), and rcu/rcustats (which internal counters for debugging RCU).
  20
  21 The output of "cat rcu/rcuctrs" looks as follows:
  22
  23 CPU last cur F M
  24   0    5  -5 0 0
  25   1   -1   0 0 0
  26   2    0   1 0 0
  27   3    0   1 0 0
  28   4    0   1 0 0
  29   5    0   1 0 0
  30   6    0   2 0 0
  31   7    0  -1 0 0
  32   8    0   1 0 0
  33 ggp = 26226, state = waitzero
  34
  35 The per-CPU fields are as follows:
  36
  37 o       "CPU" gives the CPU number.  Offline CPUs are not displayed.
  38
  39 o       "last" gives the value of the counter that is being decremented
  40         for the current grace period phase.  In the example above,
  41         the counters sum to 4, indicating that there are still four
  42         RCU read-side critical sections still running that started
  43         before the last counter flip.
  44
  45 o       "cur" gives the value of the counter that is currently being
  46         both incremented (by rcu_read_lock()) and decremented (by
  47         rcu_read_unlock()).  In the example above, the counters sum to
  48         1, indicating that there is only one RCU read-side critical section
  49         still running that started after the last counter flip.
  50
  51 o       "F" indicates whether RCU is waiting for this CPU to acknowledge
  52         a counter flip.  In the above example, RCU is not waiting on any,
  53         which is consistent with the state being "waitzero" rather than
  54         "waitack".
  55
  56 o       "M" indicates whether RCU is waiting for this CPU to execute a
  57         memory barrier.  In the above example, RCU is not waiting on any,
  58         which is consistent with the state being "waitzero" rather than
  59         "waitmb".
  60
  61 o       "ggp" is the global grace-period counter.
  62
  63 o       "state" is the RCU state, which can be one of the following:
  64
  65         o       "idle": there is no grace period in progress.
  66
  67         o       "waitack": RCU just incremented the global grace-period
  68                 counter, which has the effect of reversing the roles of
  69                 the "last" and "cur" counters above, and is waiting for
  70                 all the CPUs to acknowledge the flip.  Once the flip has
  71                 been acknowledged, CPUs will no longer be incrementing
  72                 what are now the "last" counters, so that their sum will
  73                 decrease monotonically down to zero.
  74
  75         o       "waitzero": RCU is waiting for the sum of the "last" counters
  76                 to decrease to zero.
  77
  78         o       "waitmb": RCU is waiting for each CPU to execute a memory
  79                 barrier, which ensures that instructions from a given CPU's
  80                 last RCU read-side critical section cannot be reordered
  81                 with instructions following the memory-barrier instruction.
  82
  83 The output of "cat rcu/rcugp" looks as follows:
  84
  85 oldggp=48870  newggp=48873
  86
  87 Note that reading from this file provokes a synchronize_rcu().  The
  88 "oldggp" value is that of "ggp" from rcu/rcuctrs above, taken before
  89 executing the synchronize_rcu(), and the "newggp" value is also the
  90 "ggp" value, but taken after the synchronize_rcu() command returns.
  91
  92
  93 The output of "cat rcu/rcugp" looks as follows:
  94
  95 na=1337955 nl=40 wa=1337915 wl=44 da=1337871 dl=0 dr=1337871 di=1337871
  96 1=50989 e1=6138 i1=49722 ie1=82 g1=49640 a1=315203 ae1=265563 a2=49640
  97 z1=1401244 ze1=1351605 z2=49639 m1=5661253 me1=5611614 m2=49639
  98
  99 These are counters tracking internal preemptable-RCU events, however,
 100 some of them may be useful for debugging algorithms using RCU.  In
 101 particular, the "nl", "wl", and "dl" values track the number of RCU
 102 callbacks in various states.  The fields are as follows:
 103
 104 o       "na" is the total number of RCU callbacks that have been enqueued
 105         since boot.
 106
 107 o       "nl" is the number of RCU callbacks waiting for the previous
 108         grace period to end so that they can start waiting on the next
 109         grace period.
 110
 111 o       "wa" is the total number of RCU callbacks that have started waiting
 112         for a grace period since boot.  "na" should be roughly equal to
 113         "nl" plus "wa".
 114
 115 o       "wl" is the number of RCU callbacks currently waiting for their
 116         grace period to end.
 117
 118 o       "da" is the total number of RCU callbacks whose grace periods
 119         have completed since boot.  "wa" should be roughly equal to
 120         "wl" plus "da".
 121
 122 o       "dr" is the total number of RCU callbacks that have been removed
 123         from the list of callbacks ready to invoke.  "dr" should be roughly
 124         equal to "da".
 125
 126 o       "di" is the total number of RCU callbacks that have been invoked
 127         since boot.  "di" should be roughly equal to "da", though some
 128         early versions of preemptable RCU had a bug so that only the
 129         last CPU's count of invocations was displayed, rather than the
 130         sum of all CPU's counts.
 131
 132 o       "1" is the number of calls to rcu_try_flip().  This should be
 133         roughly equal to the sum of "e1", "i1", "a1", "z1", and "m1"
 134         described below.  In other words, the number of times that
 135         the state machine is visited should be equal to the sum of the
 136         number of times that each state is visited plus the number of
 137         times that the state-machine lock acquisition failed.
 138
 139 o       "e1" is the number of times that rcu_try_flip() was unable to
 140         acquire the fliplock.
 141
 142 o       "i1" is the number of calls to rcu_try_flip_idle().
 143
 144 o       "ie1" is the number of times rcu_try_flip_idle() exited early
 145         due to the calling CPU having no work for RCU.
 146
 147 o       "g1" is the number of times that rcu_try_flip_idle() decided
 148         to start a new grace period.  "i1" should be roughly equal to
 149         "ie1" plus "g1".
 150
 151 o       "a1" is the number of calls to rcu_try_flip_waitack().
 152
 153 o       "ae1" is the number of times that rcu_try_flip_waitack() found
 154         that at least one CPU had not yet acknowledge the new grace period
 155         (AKA "counter flip").
 156
 157 o       "a2" is the number of time rcu_try_flip_waitack() found that
 158         all CPUs had acknowledged.  "a1" should be roughly equal to
 159         "ae1" plus "a2".  (This particular output was collected on
 160         a 128-CPU machine, hence the smaller-than-usual fraction of
 161         calls to rcu_try_flip_waitack() finding all CPUs having already
 162         acknowledged.)
 163
 164 o       "z1" is the number of calls to rcu_try_flip_waitzero().
 165
 166 o       "ze1" is the number of times that rcu_try_flip_waitzero() found
 167         that not all of the old RCU read-side critical sections had
 168         completed.
 169
 170 o       "z2" is the number of times that rcu_try_flip_waitzero() finds
 171         the sum of the counters equal to zero, in other words, that
 172         all of the old RCU read-side critical sections had completed.
 173         The value of "z1" should be roughly equal to "ze1" plus
 174         "z2".
 175
 176 o       "m1" is the number of calls to rcu_try_flip_waitmb().
 177
 178 o       "me1" is the number of times that rcu_try_flip_waitmb() finds
 179         that at least one CPU has not yet executed a memory barrier.
 180
 181 o       "m2" is the number of times that rcu_try_flip_waitmb() finds that
 182         all CPUs have executed a memory barrier.
 183
 184
 185 Hierarchical RCU debugfs Files and Formats
 186
 187 This implementation of RCU provides three debugfs files under the
 188 top-level directory RCU: rcu/rcudata (which displays fields in struct
 189 rcu_data), rcu/rcugp (which displays grace-period counters), and
 190 rcu/rcuhier (which displays the struct rcu_node hierarchy).
 191
 192 The output of "cat rcu/rcudata" looks as follows:
 193
 194 rcu_sched:
 195   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
 196   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
 197   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
 198   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
 199   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
 200   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
 201   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
 202   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
 203 rcu_bh:
 204   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
 205   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
 206   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
 207   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
 208   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
 209   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
 210   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
 211   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
 212
 213 The first section lists the rcu_data structures for rcu, the second for
 214 rcu_bh.  Each section has one line per CPU, or eight for this 8-CPU system.
 215 The fields are as follows:
 216
 217 o       The number at the beginning of each line is the CPU number.
 218         CPUs numbers followed by an exclamation mark are offline,
 219         but have been online at least once since boot.  There will be
 220         no output for CPUs that have never been online, which can be
 221         a good thing in the surprisingly common case where NR_CPUS is
 222         substantially larger than the number of actual CPUs.
 223
 224 o       "c" is the count of grace periods that this CPU believes have
 225         completed.  CPUs in dynticks idle mode may lag quite a ways
 226         behind, for example, CPU 4 under "rcu" above, which has slept
 227         through the past 25 RCU grace periods.  It is not unusual to
 228         see CPUs lagging by thousands of grace periods.
 229
 230 o       "g" is the count of grace periods that this CPU believes have
 231         started.  Again, CPUs in dynticks idle mode may lag behind.
 232         If the "c" and "g" values are equal, this CPU has already
 233         reported a quiescent state for the last RCU grace period that
 234         it is aware of, otherwise, the CPU believes that it owes RCU a
 235         quiescent state.
 236
 237 o       "pq" indicates that this CPU has passed through a quiescent state
 238         for the current grace period.  It is possible for "pq" to be
 239         "1" and "c" different than "g", which indicates that although
 240         the CPU has passed through a quiescent state, either (1) this
 241         CPU has not yet reported that fact, (2) some other CPU has not
 242         yet reported for this grace period, or (3) both.
 243
 244 o       "pqc" indicates which grace period the last-observed quiescent
 245         state for this CPU corresponds to.  This is important for handling
 246         the race between CPU 0 reporting an extended dynticks-idle
 247         quiescent state for CPU 1 and CPU 1 suddenly waking up and
 248         reporting its own quiescent state.  If CPU 1 was the last CPU
 249         for the current grace period, then the CPU that loses this race
 250         will attempt to incorrectly mark CPU 1 as having checked in for
 251         the next grace period!
 252
 253 o       "qp" indicates that RCU still expects a quiescent state from
 254         this CPU.
 255
 256 o       "dt" is the current value of the dyntick counter that is incremented
 257         when entering or leaving dynticks idle state, either by the
 258         scheduler or by irq.  The number after the "/" is the interrupt
 259         nesting depth when in dyntick-idle state, or one greater than
 260         the interrupt-nesting depth otherwise.
 261
 262         This field is displayed only for CONFIG_NO_HZ kernels.
 263
 264 o       "dn" is the current value of the dyntick counter that is incremented
 265         when entering or leaving dynticks idle state via NMI.  If both
 266         the "dt" and "dn" values are even, then this CPU is in dynticks
 267         idle mode and may be ignored by RCU.  If either of these two
 268         counters is odd, then RCU must be alert to the possibility of
 269         an RCU read-side critical section running on this CPU.
 270
 271         This field is displayed only for CONFIG_NO_HZ kernels.
 272
 273 o       "df" is the number of times that some other CPU has forced a
 274         quiescent state on behalf of this CPU due to this CPU being in
 275         dynticks-idle state.
 276
 277         This field is displayed only for CONFIG_NO_HZ kernels.
 278
 279 o       "of" is the number of times that some other CPU has forced a
 280         quiescent state on behalf of this CPU due to this CPU being
 281         offline.  In a perfect world, this might neve happen, but it
 282         turns out that offlining and onlining a CPU can take several grace
 283         periods, and so there is likely to be an extended period of time
 284         when RCU believes that the CPU is online when it really is not.
 285         Please note that erring in the other direction (RCU believing a
 286         CPU is offline when it is really alive and kicking) is a fatal
 287         error, so it makes sense to err conservatively.
 288
 289 o       "ri" is the number of times that RCU has seen fit to send a
 290         reschedule IPI to this CPU in order to get it to report a
 291         quiescent state.
 292
 293 o       "ql" is the number of RCU callbacks currently residing on
 294         this CPU.  This is the total number of callbacks, regardless
 295         of what state they are in (new, waiting for grace period to
 296         start, waiting for grace period to end, ready to invoke).
 297
 298 o       "b" is the batch limit for this CPU.  If more than this number
 299         of RCU callbacks is ready to invoke, then the remainder will
 300         be deferred.
 301
 302 There is also an rcu/rcudata.csv file with the same information in
 303 comma-separated-variable spreadsheet format.
 304
 305
 306 The output of "cat rcu/rcugp" looks as follows:
 307
 308 rcu_sched: completed=33062  gpnum=33063
 309 rcu_bh: completed=464  gpnum=464
 310
 311 Again, this output is for both "rcu" and "rcu_bh".  The fields are
 312 taken from the rcu_state structure, and are as follows:
 313
 314 o       "completed" is the number of grace periods that have completed.
 315         It is comparable to the "c" field from rcu/rcudata in that a
 316         CPU whose "c" field matches the value of "completed" is aware
 317         that the corresponding RCU grace period has completed.
 318
 319 o       "gpnum" is the number of grace periods that have started.  It is
 320         comparable to the "g" field from rcu/rcudata in that a CPU
 321         whose "g" field matches the value of "gpnum" is aware that the
 322         corresponding RCU grace period has started.
 323
 324         If these two fields are equal (as they are for "rcu_bh" above),
 325         then there is no grace period in progress, in other words, RCU
 326         is idle.  On the other hand, if the two fields differ (as they
 327         do for "rcu" above), then an RCU grace period is in progress.
 328
 329
 330 The output of "cat rcu/rcuhier" looks as follows, with very long lines:
 331
 332 c=6902 g=6903 s=2 jfq=3 j=72c7 nfqs=13142/nfqsng=0(13142) fqlh=6
 333 1/1 0:127 ^0
 334 3/3 0:35 ^0    0/0 36:71 ^1    0/0 72:107 ^2    0/0 108:127 ^3
 335 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
 336 rcu_bh:
 337 c=-226 g=-226 s=1 jfq=-5701 j=72c7 nfqs=88/nfqsng=0(88) fqlh=0
 338 0/1 0:127 ^0
 339 0/3 0:35 ^0    0/0 36:71 ^1    0/0 72:107 ^2    0/0 108:127 ^3
 340 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
 341
 342 This is once again split into "rcu" and "rcu_bh" portions.  The fields are
 343 as follows:
 344
 345 o       "c" is exactly the same as "completed" under rcu/rcugp.
 346
 347 o       "g" is exactly the same as "gpnum" under rcu/rcugp.
 348
 349 o       "s" is the "signaled" state that drives force_quiescent_state()'s
 350         state machine.
 351
 352 o       "jfq" is the number of jiffies remaining for this grace period
 353         before force_quiescent_state() is invoked to help push things
 354         along.  Note that CPUs in dyntick-idle mode thoughout the grace
 355         period will not report on their own, but rather must be check by
 356         some other CPU via force_quiescent_state().
 357
 358 o       "j" is the low-order four hex digits of the jiffies counter.
 359         Yes, Paul did run into a number of problems that turned out to
 360         be due to the jiffies counter no longer counting.  Why do you ask?
 361
 362 o       "nfqs" is the number of calls to force_quiescent_state() since
 363         boot.
 364
 365 o       "nfqsng" is the number of useless calls to force_quiescent_state(),
 366         where there wasn't actually a grace period active.  This can
 367         happen due to races.  The number in parentheses is the difference
 368         between "nfqs" and "nfqsng", or the number of times that
 369         force_quiescent_state() actually did some real work.
 370
 371 o       "fqlh" is the number of calls to force_quiescent_state() that
 372         exited immediately (without even being counted in nfqs above)
 373         due to contention on ->fqslock.
 374
 375 o       Each element of the form "1/1 0:127 ^0" represents one struct
 376         rcu_node.  Each line represents one level of the hierarchy, from
 377         root to leaves.  It is best to think of the rcu_data structures
 378         as forming yet another level after the leaves.  Note that there
 379         might be either one, two, or three levels of rcu_node structures,
 380         depending on the relationship between CONFIG_RCU_FANOUT and
 381         CONFIG_NR_CPUS.
 382
 383         o       The numbers separated by the "/" are the qsmask followed
 384                 by the qsmaskinit.  The qsmask will have one bit
 385                 set for each entity in the next lower level that
 386                 has not yet checked in for the current grace period.
 387                 The qsmaskinit will have one bit for each entity that is
 388                 currently expected to check in during each grace period.
 389                 The value of qsmaskinit is assigned to that of qsmask
 390                 at the beginning of each grace period.
 391
 392                 For example, for "rcu", the qsmask of the first entry
 393                 of the lowest level is 0x14, meaning that we are still
 394                 waiting for CPUs 2 and 4 to check in for the current
 395                 grace period.
 396
 397         o       The numbers separated by the ":" are the range of CPUs
 398                 served by this struct rcu_node.  This can be helpful
 399                 in working out how the hierarchy is wired together.
 400
 401                 For example, the first entry at the lowest level shows
 402                 "0:5", indicating that it covers CPUs 0 through 5.
 403
 404         o       The number after the "^" indicates the bit in the
 405                 next higher level rcu_node structure that this
 406                 rcu_node structure corresponds to.
 407
 408                 For example, the first entry at the lowest level shows
 409                 "^0", indicating that it corresponds to bit zero in
 410                 the first entry at the middle level.
 411
 412
 413 The output of "cat rcu/rcu_pending" looks as follows:
 414
 415 rcu_sched:
 416   0 np=255892 qsp=53936 cbr=0 cng=14417 gpc=10033 gps=24320 nf=6445 nn=146741
 417   1 np=261224 qsp=54638 cbr=0 cng=25723 gpc=16310 gps=2849 nf=5912 nn=155792
 418   2 np=237496 qsp=49664 cbr=0 cng=2762 gpc=45478 gps=1762 nf=1201 nn=136629
 419   3 np=236249 qsp=48766 cbr=0 cng=286 gpc=48049 gps=1218 nf=207 nn=137723
 420   4 np=221310 qsp=46850 cbr=0 cng=26 gpc=43161 gps=4634 nf=3529 nn=123110
 421   5 np=237332 qsp=48449 cbr=0 cng=54 gpc=47920 gps=3252 nf=201 nn=137456
 422   6 np=219995 qsp=46718 cbr=0 cng=50 gpc=42098 gps=6093 nf=4202 nn=120834
 423   7 np=249893 qsp=49390 cbr=0 cng=72 gpc=38400 gps=17102 nf=41 nn=144888
 424 rcu_bh:
 425   0 np=146741 qsp=1419 cbr=0 cng=6 gpc=0 gps=0 nf=2 nn=145314
 426   1 np=155792 qsp=12597 cbr=0 cng=0 gpc=4 gps=8 nf=3 nn=143180
 427   2 np=136629 qsp=18680 cbr=0 cng=0 gpc=7 gps=6 nf=0 nn=117936
 428   3 np=137723 qsp=2843 cbr=0 cng=0 gpc=10 gps=7 nf=0 nn=134863
 429   4 np=123110 qsp=12433 cbr=0 cng=0 gpc=4 gps=2 nf=0 nn=110671
 430   5 np=137456 qsp=4210 cbr=0 cng=0 gpc=6 gps=5 nf=0 nn=133235
 431   6 np=120834 qsp=9902 cbr=0 cng=0 gpc=6 gps=3 nf=2 nn=110921
 432   7 np=144888 qsp=26336 cbr=0 cng=0 gpc=8 gps=2 nf=0 nn=118542
 433
 434 As always, this is once again split into "rcu" and "rcu_bh" portions.
 435 The fields are as follows:
 436
 437 o       "np" is the number of times that __rcu_pending() has been invoked
 438         for the corresponding flavor of RCU.
 439
 440 o       "qsp" is the number of times that the RCU was waiting for a
 441         quiescent state from this CPU.
 442
 443 o       "cbr" is the number of times that this CPU had RCU callbacks
 444         that had passed through a grace period, and were thus ready
 445         to be invoked.
 446
 447 o       "cng" is the number of times that this CPU needed another
 448         grace period while RCU was idle.
 449
 450 o       "gpc" is the number of times that an old grace period had
 451         completed, but this CPU was not yet aware of it.
 452
 453 o       "gps" is the number of times that a new grace period had started,
 454         but this CPU was not yet aware of it.
 455
 456 o       "nf" is the number of times that this CPU suspected that the
 457         current grace period had run for too long, and thus needed to
 458         be forced.
 459
 460         Please note that "forcing" consists of sending resched IPIs
 461         to holdout CPUs.  If that CPU really still is in an old RCU
 462         read-side critical section, then we really do have to wait for it.
 463         The assumption behing "forcing" is that the CPU is not still in
 464         an old RCU read-side critical section, but has not yet responded
 465         for some other reason.
 466
 467 o       "nn" is the number of times that this CPU needed nothing.  Alert
 468         readers will note that the rcu "nn" number for a given CPU very
 469         closely matches the rcu_bh "np" number for that same CPU.  This
 470         is due to short-circuit evaluation in rcu_pending().