472 lines
20 KiB
Plaintext
472 lines
20 KiB
Plaintext
CONFIG_RCU_TRACE debugfs Files and Formats
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The rcupreempt and rcutree implementations of RCU provide debugfs trace
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output that summarizes counters and state. This information is useful for
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debugging RCU itself, and can sometimes also help to debug abuses of RCU.
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Note that the rcuclassic implementation of RCU does not provide debugfs
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trace output.
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The following sections describe the debugfs files and formats for
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preemptable RCU (rcupreempt) and hierarchical RCU (rcutree).
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Preemptable RCU debugfs Files and Formats
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This implementation of RCU provides three debugfs files under the
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top-level directory RCU: rcu/rcuctrs (which displays the per-CPU
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counters used by preemptable RCU) rcu/rcugp (which displays grace-period
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counters), and rcu/rcustats (which internal counters for debugging RCU).
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The output of "cat rcu/rcuctrs" looks as follows:
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CPU last cur F M
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0 5 -5 0 0
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1 -1 0 0 0
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2 0 1 0 0
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3 0 1 0 0
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4 0 1 0 0
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5 0 1 0 0
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6 0 2 0 0
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7 0 -1 0 0
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8 0 1 0 0
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ggp = 26226, state = waitzero
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The per-CPU fields are as follows:
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o "CPU" gives the CPU number. Offline CPUs are not displayed.
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o "last" gives the value of the counter that is being decremented
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for the current grace period phase. In the example above,
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the counters sum to 4, indicating that there are still four
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RCU read-side critical sections still running that started
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before the last counter flip.
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o "cur" gives the value of the counter that is currently being
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both incremented (by rcu_read_lock()) and decremented (by
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rcu_read_unlock()). In the example above, the counters sum to
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1, indicating that there is only one RCU read-side critical section
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still running that started after the last counter flip.
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o "F" indicates whether RCU is waiting for this CPU to acknowledge
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a counter flip. In the above example, RCU is not waiting on any,
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which is consistent with the state being "waitzero" rather than
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"waitack".
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o "M" indicates whether RCU is waiting for this CPU to execute a
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memory barrier. In the above example, RCU is not waiting on any,
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which is consistent with the state being "waitzero" rather than
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"waitmb".
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o "ggp" is the global grace-period counter.
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o "state" is the RCU state, which can be one of the following:
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o "idle": there is no grace period in progress.
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o "waitack": RCU just incremented the global grace-period
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counter, which has the effect of reversing the roles of
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the "last" and "cur" counters above, and is waiting for
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all the CPUs to acknowledge the flip. Once the flip has
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been acknowledged, CPUs will no longer be incrementing
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what are now the "last" counters, so that their sum will
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decrease monotonically down to zero.
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o "waitzero": RCU is waiting for the sum of the "last" counters
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to decrease to zero.
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o "waitmb": RCU is waiting for each CPU to execute a memory
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barrier, which ensures that instructions from a given CPU's
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last RCU read-side critical section cannot be reordered
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with instructions following the memory-barrier instruction.
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The output of "cat rcu/rcugp" looks as follows:
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oldggp=48870 newggp=48873
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Note that reading from this file provokes a synchronize_rcu(). The
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"oldggp" value is that of "ggp" from rcu/rcuctrs above, taken before
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executing the synchronize_rcu(), and the "newggp" value is also the
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"ggp" value, but taken after the synchronize_rcu() command returns.
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The output of "cat rcu/rcugp" looks as follows:
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na=1337955 nl=40 wa=1337915 wl=44 da=1337871 dl=0 dr=1337871 di=1337871
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1=50989 e1=6138 i1=49722 ie1=82 g1=49640 a1=315203 ae1=265563 a2=49640
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z1=1401244 ze1=1351605 z2=49639 m1=5661253 me1=5611614 m2=49639
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These are counters tracking internal preemptable-RCU events, however,
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some of them may be useful for debugging algorithms using RCU. In
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particular, the "nl", "wl", and "dl" values track the number of RCU
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callbacks in various states. The fields are as follows:
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o "na" is the total number of RCU callbacks that have been enqueued
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since boot.
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o "nl" is the number of RCU callbacks waiting for the previous
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grace period to end so that they can start waiting on the next
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grace period.
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o "wa" is the total number of RCU callbacks that have started waiting
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for a grace period since boot. "na" should be roughly equal to
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"nl" plus "wa".
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o "wl" is the number of RCU callbacks currently waiting for their
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grace period to end.
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o "da" is the total number of RCU callbacks whose grace periods
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have completed since boot. "wa" should be roughly equal to
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"wl" plus "da".
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o "dr" is the total number of RCU callbacks that have been removed
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from the list of callbacks ready to invoke. "dr" should be roughly
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equal to "da".
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o "di" is the total number of RCU callbacks that have been invoked
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since boot. "di" should be roughly equal to "da", though some
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early versions of preemptable RCU had a bug so that only the
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last CPU's count of invocations was displayed, rather than the
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sum of all CPU's counts.
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o "1" is the number of calls to rcu_try_flip(). This should be
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roughly equal to the sum of "e1", "i1", "a1", "z1", and "m1"
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described below. In other words, the number of times that
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the state machine is visited should be equal to the sum of the
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number of times that each state is visited plus the number of
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times that the state-machine lock acquisition failed.
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o "e1" is the number of times that rcu_try_flip() was unable to
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acquire the fliplock.
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o "i1" is the number of calls to rcu_try_flip_idle().
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o "ie1" is the number of times rcu_try_flip_idle() exited early
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due to the calling CPU having no work for RCU.
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o "g1" is the number of times that rcu_try_flip_idle() decided
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to start a new grace period. "i1" should be roughly equal to
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"ie1" plus "g1".
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o "a1" is the number of calls to rcu_try_flip_waitack().
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o "ae1" is the number of times that rcu_try_flip_waitack() found
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that at least one CPU had not yet acknowledge the new grace period
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(AKA "counter flip").
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o "a2" is the number of time rcu_try_flip_waitack() found that
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all CPUs had acknowledged. "a1" should be roughly equal to
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"ae1" plus "a2". (This particular output was collected on
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a 128-CPU machine, hence the smaller-than-usual fraction of
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calls to rcu_try_flip_waitack() finding all CPUs having already
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acknowledged.)
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o "z1" is the number of calls to rcu_try_flip_waitzero().
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o "ze1" is the number of times that rcu_try_flip_waitzero() found
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that not all of the old RCU read-side critical sections had
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completed.
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o "z2" is the number of times that rcu_try_flip_waitzero() finds
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the sum of the counters equal to zero, in other words, that
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all of the old RCU read-side critical sections had completed.
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The value of "z1" should be roughly equal to "ze1" plus
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"z2".
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o "m1" is the number of calls to rcu_try_flip_waitmb().
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o "me1" is the number of times that rcu_try_flip_waitmb() finds
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that at least one CPU has not yet executed a memory barrier.
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o "m2" is the number of times that rcu_try_flip_waitmb() finds that
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all CPUs have executed a memory barrier.
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Hierarchical RCU debugfs Files and Formats
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This implementation of RCU provides three debugfs files under the
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top-level directory RCU: rcu/rcudata (which displays fields in struct
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rcu_data), rcu/rcugp (which displays grace-period counters), and
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rcu/rcuhier (which displays the struct rcu_node hierarchy).
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The output of "cat rcu/rcudata" looks as follows:
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rcu:
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rcu:
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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
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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
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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
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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
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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
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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
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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
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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
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rcu_bh:
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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
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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
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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
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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
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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
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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
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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
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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
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The first section lists the rcu_data structures for rcu, the second for
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rcu_bh. Each section has one line per CPU, or eight for this 8-CPU system.
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The fields are as follows:
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o The number at the beginning of each line is the CPU number.
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CPUs numbers followed by an exclamation mark are offline,
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but have been online at least once since boot. There will be
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no output for CPUs that have never been online, which can be
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a good thing in the surprisingly common case where NR_CPUS is
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substantially larger than the number of actual CPUs.
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o "c" is the count of grace periods that this CPU believes have
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completed. CPUs in dynticks idle mode may lag quite a ways
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behind, for example, CPU 4 under "rcu" above, which has slept
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through the past 25 RCU grace periods. It is not unusual to
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see CPUs lagging by thousands of grace periods.
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o "g" is the count of grace periods that this CPU believes have
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started. Again, CPUs in dynticks idle mode may lag behind.
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If the "c" and "g" values are equal, this CPU has already
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reported a quiescent state for the last RCU grace period that
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it is aware of, otherwise, the CPU believes that it owes RCU a
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quiescent state.
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o "pq" indicates that this CPU has passed through a quiescent state
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for the current grace period. It is possible for "pq" to be
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"1" and "c" different than "g", which indicates that although
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the CPU has passed through a quiescent state, either (1) this
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CPU has not yet reported that fact, (2) some other CPU has not
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yet reported for this grace period, or (3) both.
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o "pqc" indicates which grace period the last-observed quiescent
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state for this CPU corresponds to. This is important for handling
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the race between CPU 0 reporting an extended dynticks-idle
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quiescent state for CPU 1 and CPU 1 suddenly waking up and
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reporting its own quiescent state. If CPU 1 was the last CPU
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for the current grace period, then the CPU that loses this race
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will attempt to incorrectly mark CPU 1 as having checked in for
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the next grace period!
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o "qp" indicates that RCU still expects a quiescent state from
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this CPU.
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o "dt" is the current value of the dyntick counter that is incremented
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when entering or leaving dynticks idle state, either by the
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scheduler or by irq. The number after the "/" is the interrupt
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nesting depth when in dyntick-idle state, or one greater than
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the interrupt-nesting depth otherwise.
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This field is displayed only for CONFIG_NO_HZ kernels.
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o "dn" is the current value of the dyntick counter that is incremented
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when entering or leaving dynticks idle state via NMI. If both
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the "dt" and "dn" values are even, then this CPU is in dynticks
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idle mode and may be ignored by RCU. If either of these two
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counters is odd, then RCU must be alert to the possibility of
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an RCU read-side critical section running on this CPU.
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This field is displayed only for CONFIG_NO_HZ kernels.
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o "df" is the number of times that some other CPU has forced a
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quiescent state on behalf of this CPU due to this CPU being in
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dynticks-idle state.
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This field is displayed only for CONFIG_NO_HZ kernels.
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o "of" is the number of times that some other CPU has forced a
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quiescent state on behalf of this CPU due to this CPU being
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offline. In a perfect world, this might neve happen, but it
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turns out that offlining and onlining a CPU can take several grace
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periods, and so there is likely to be an extended period of time
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when RCU believes that the CPU is online when it really is not.
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Please note that erring in the other direction (RCU believing a
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CPU is offline when it is really alive and kicking) is a fatal
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error, so it makes sense to err conservatively.
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o "ri" is the number of times that RCU has seen fit to send a
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reschedule IPI to this CPU in order to get it to report a
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quiescent state.
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o "ql" is the number of RCU callbacks currently residing on
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this CPU. This is the total number of callbacks, regardless
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of what state they are in (new, waiting for grace period to
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start, waiting for grace period to end, ready to invoke).
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o "b" is the batch limit for this CPU. If more than this number
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of RCU callbacks is ready to invoke, then the remainder will
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be deferred.
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There is also an rcu/rcudata.csv file with the same information in
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comma-separated-variable spreadsheet format.
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The output of "cat rcu/rcugp" looks as follows:
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rcu: completed=33062 gpnum=33063
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rcu_bh: completed=464 gpnum=464
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Again, this output is for both "rcu" and "rcu_bh". The fields are
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taken from the rcu_state structure, and are as follows:
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o "completed" is the number of grace periods that have completed.
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It is comparable to the "c" field from rcu/rcudata in that a
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CPU whose "c" field matches the value of "completed" is aware
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that the corresponding RCU grace period has completed.
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o "gpnum" is the number of grace periods that have started. It is
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comparable to the "g" field from rcu/rcudata in that a CPU
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whose "g" field matches the value of "gpnum" is aware that the
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corresponding RCU grace period has started.
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If these two fields are equal (as they are for "rcu_bh" above),
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then there is no grace period in progress, in other words, RCU
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is idle. On the other hand, if the two fields differ (as they
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do for "rcu" above), then an RCU grace period is in progress.
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The output of "cat rcu/rcuhier" looks as follows, with very long lines:
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c=6902 g=6903 s=2 jfq=3 j=72c7 nfqs=13142/nfqsng=0(13142) fqlh=6
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1/1 0:127 ^0
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3/3 0:35 ^0 0/0 36:71 ^1 0/0 72:107 ^2 0/0 108:127 ^3
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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
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rcu_bh:
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c=-226 g=-226 s=1 jfq=-5701 j=72c7 nfqs=88/nfqsng=0(88) fqlh=0
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0/1 0:127 ^0
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0/3 0:35 ^0 0/0 36:71 ^1 0/0 72:107 ^2 0/0 108:127 ^3
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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
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This is once again split into "rcu" and "rcu_bh" portions. The fields are
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as follows:
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o "c" is exactly the same as "completed" under rcu/rcugp.
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o "g" is exactly the same as "gpnum" under rcu/rcugp.
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o "s" is the "signaled" state that drives force_quiescent_state()'s
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state machine.
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o "jfq" is the number of jiffies remaining for this grace period
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before force_quiescent_state() is invoked to help push things
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along. Note that CPUs in dyntick-idle mode thoughout the grace
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period will not report on their own, but rather must be check by
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some other CPU via force_quiescent_state().
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o "j" is the low-order four hex digits of the jiffies counter.
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Yes, Paul did run into a number of problems that turned out to
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be due to the jiffies counter no longer counting. Why do you ask?
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o "nfqs" is the number of calls to force_quiescent_state() since
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boot.
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o "nfqsng" is the number of useless calls to force_quiescent_state(),
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where there wasn't actually a grace period active. This can
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happen due to races. The number in parentheses is the difference
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between "nfqs" and "nfqsng", or the number of times that
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force_quiescent_state() actually did some real work.
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o "fqlh" is the number of calls to force_quiescent_state() that
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exited immediately (without even being counted in nfqs above)
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due to contention on ->fqslock.
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o Each element of the form "1/1 0:127 ^0" represents one struct
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rcu_node. Each line represents one level of the hierarchy, from
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root to leaves. It is best to think of the rcu_data structures
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as forming yet another level after the leaves. Note that there
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might be either one, two, or three levels of rcu_node structures,
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depending on the relationship between CONFIG_RCU_FANOUT and
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CONFIG_NR_CPUS.
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o The numbers separated by the "/" are the qsmask followed
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by the qsmaskinit. The qsmask will have one bit
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set for each entity in the next lower level that
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has not yet checked in for the current grace period.
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The qsmaskinit will have one bit for each entity that is
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currently expected to check in during each grace period.
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The value of qsmaskinit is assigned to that of qsmask
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at the beginning of each grace period.
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For example, for "rcu", the qsmask of the first entry
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of the lowest level is 0x14, meaning that we are still
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waiting for CPUs 2 and 4 to check in for the current
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grace period.
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o The numbers separated by the ":" are the range of CPUs
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served by this struct rcu_node. This can be helpful
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in working out how the hierarchy is wired together.
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For example, the first entry at the lowest level shows
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"0:5", indicating that it covers CPUs 0 through 5.
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o The number after the "^" indicates the bit in the
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next higher level rcu_node structure that this
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rcu_node structure corresponds to.
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For example, the first entry at the lowest level shows
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"^0", indicating that it corresponds to bit zero in
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the first entry at the middle level.
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The output of "cat rcu/rcu_pending" looks as follows:
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rcu:
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0 np=255892 qsp=53936 cbr=0 cng=14417 gpc=10033 gps=24320 nf=6445 nn=146741
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1 np=261224 qsp=54638 cbr=0 cng=25723 gpc=16310 gps=2849 nf=5912 nn=155792
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2 np=237496 qsp=49664 cbr=0 cng=2762 gpc=45478 gps=1762 nf=1201 nn=136629
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3 np=236249 qsp=48766 cbr=0 cng=286 gpc=48049 gps=1218 nf=207 nn=137723
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4 np=221310 qsp=46850 cbr=0 cng=26 gpc=43161 gps=4634 nf=3529 nn=123110
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5 np=237332 qsp=48449 cbr=0 cng=54 gpc=47920 gps=3252 nf=201 nn=137456
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6 np=219995 qsp=46718 cbr=0 cng=50 gpc=42098 gps=6093 nf=4202 nn=120834
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7 np=249893 qsp=49390 cbr=0 cng=72 gpc=38400 gps=17102 nf=41 nn=144888
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rcu_bh:
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0 np=146741 qsp=1419 cbr=0 cng=6 gpc=0 gps=0 nf=2 nn=145314
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1 np=155792 qsp=12597 cbr=0 cng=0 gpc=4 gps=8 nf=3 nn=143180
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2 np=136629 qsp=18680 cbr=0 cng=0 gpc=7 gps=6 nf=0 nn=117936
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3 np=137723 qsp=2843 cbr=0 cng=0 gpc=10 gps=7 nf=0 nn=134863
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4 np=123110 qsp=12433 cbr=0 cng=0 gpc=4 gps=2 nf=0 nn=110671
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5 np=137456 qsp=4210 cbr=0 cng=0 gpc=6 gps=5 nf=0 nn=133235
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6 np=120834 qsp=9902 cbr=0 cng=0 gpc=6 gps=3 nf=2 nn=110921
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7 np=144888 qsp=26336 cbr=0 cng=0 gpc=8 gps=2 nf=0 nn=118542
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As always, this is once again split into "rcu" and "rcu_bh" portions.
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The fields are as follows:
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o "np" is the number of times that __rcu_pending() has been invoked
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for the corresponding flavor of RCU.
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o "qsp" is the number of times that the RCU was waiting for a
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quiescent state from this CPU.
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o "cbr" is the number of times that this CPU had RCU callbacks
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that had passed through a grace period, and were thus ready
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to be invoked.
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o "cng" is the number of times that this CPU needed another
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grace period while RCU was idle.
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o "gpc" is the number of times that an old grace period had
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completed, but this CPU was not yet aware of it.
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o "gps" is the number of times that a new grace period had started,
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but this CPU was not yet aware of it.
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o "nf" is the number of times that this CPU suspected that the
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current grace period had run for too long, and thus needed to
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be forced.
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Please note that "forcing" consists of sending resched IPIs
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to holdout CPUs. If that CPU really still is in an old RCU
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read-side critical section, then we really do have to wait for it.
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The assumption behing "forcing" is that the CPU is not still in
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an old RCU read-side critical section, but has not yet responded
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for some other reason.
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o "nn" is the number of times that this CPU needed nothing. Alert
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readers will note that the rcu "nn" number for a given CPU very
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closely matches the rcu_bh "np" number for that same CPU. This
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is due to short-circuit evaluation in rcu_pending().
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