[deliverable/linux.git] / Documentation / RCU / trace.txt

CONFIG_RCU_TRACE debugfs Files and Formats


The rcutree and rcutiny implementations of RCU provide debugfs trace
output that summarizes counters and state.  This information is useful for
debugging RCU itself, and can sometimes also help to debug abuses of RCU.
The following sections describe the debugfs files and formats, first
for rcutree and next for rcutiny.


CONFIG_TREE_RCU and CONFIG_TREE_PREEMPT_RCU debugfs Files and Formats

These implementations of RCU provides several debugfs files under the
top-level directory "rcu":

rcu/rcudata:
        Displays fields in struct rcu_data.
rcu/rcudata.csv:
        Comma-separated values spreadsheet version of rcudata.
rcu/rcugp:
        Displays grace-period counters.
rcu/rcuhier:
        Displays the struct rcu_node hierarchy.
rcu/rcu_pending:
        Displays counts of the reasons rcu_pending() decided that RCU had
        work to do.
rcu/rcutorture:
        Displays rcutorture test progress.
rcu/rcuboost:
        Displays RCU boosting statistics.  Only present if
        CONFIG_RCU_BOOST=y.

The output of "cat rcu/rcudata" looks as follows:

rcu_sched:
  0 c=20972 g=20973 pq=1 pqc=20972 qp=0 dt=545/1/0 df=50 of=0 ri=0 ql=163 qs=NRW. kt=0/W/0 ktl=ebc3 b=10 ci=153737 co=0 ca=0
  1 c=20972 g=20973 pq=1 pqc=20972 qp=0 dt=967/1/0 df=58 of=0 ri=0 ql=634 qs=NRW. kt=0/W/1 ktl=58c b=10 ci=191037 co=0 ca=0
  2 c=20972 g=20973 pq=1 pqc=20972 qp=0 dt=1081/1/0 df=175 of=0 ri=0 ql=74 qs=N.W. kt=0/W/2 ktl=da94 b=10 ci=75991 co=0 ca=0
  3 c=20942 g=20943 pq=1 pqc=20942 qp=1 dt=1846/0/0 df=404 of=0 ri=0 ql=0 qs=.... kt=0/W/3 ktl=d1cd b=10 ci=72261 co=0 ca=0
  4 c=20972 g=20973 pq=1 pqc=20972 qp=0 dt=369/1/0 df=83 of=0 ri=0 ql=48 qs=N.W. kt=0/W/4 ktl=e0e7 b=10 ci=128365 co=0 ca=0
  5 c=20972 g=20973 pq=1 pqc=20972 qp=0 dt=381/1/0 df=64 of=0 ri=0 ql=169 qs=NRW. kt=0/W/5 ktl=fb2f b=10 ci=164360 co=0 ca=0
  6 c=20972 g=20973 pq=1 pqc=20972 qp=0 dt=1037/1/0 df=183 of=0 ri=0 ql=62 qs=N.W. kt=0/W/6 ktl=d2ad b=10 ci=65663 co=0 ca=0
  7 c=20897 g=20897 pq=1 pqc=20896 qp=0 dt=1572/0/0 df=382 of=0 ri=0 ql=0 qs=.... kt=0/W/7 ktl=cf15 b=10 ci=75006 co=0 ca=0
rcu_bh:
  0 c=1480 g=1480 pq=1 pqc=1479 qp=0 dt=545/1/0 df=6 of=0 ri=1 ql=0 qs=.... kt=0/W/0 ktl=ebc3 b=10 ci=0 co=0 ca=0
  1 c=1480 g=1480 pq=1 pqc=1479 qp=0 dt=967/1/0 df=3 of=0 ri=1 ql=0 qs=.... kt=0/W/1 ktl=58c b=10 ci=151 co=0 ca=0
  2 c=1480 g=1480 pq=1 pqc=1479 qp=0 dt=1081/1/0 df=6 of=0 ri=1 ql=0 qs=.... kt=0/W/2 ktl=da94 b=10 ci=0 co=0 ca=0
  3 c=1480 g=1480 pq=1 pqc=1479 qp=0 dt=1846/0/0 df=8 of=0 ri=1 ql=0 qs=.... kt=0/W/3 ktl=d1cd b=10 ci=0 co=0 ca=0
  4 c=1480 g=1480 pq=1 pqc=1479 qp=0 dt=369/1/0 df=6 of=0 ri=1 ql=0 qs=.... kt=0/W/4 ktl=e0e7 b=10 ci=0 co=0 ca=0
  5 c=1480 g=1480 pq=1 pqc=1479 qp=0 dt=381/1/0 df=4 of=0 ri=1 ql=0 qs=.... kt=0/W/5 ktl=fb2f b=10 ci=0 co=0 ca=0
  6 c=1480 g=1480 pq=1 pqc=1479 qp=0 dt=1037/1/0 df=6 of=0 ri=1 ql=0 qs=.... kt=0/W/6 ktl=d2ad b=10 ci=0 co=0 ca=0
  7 c=1474 g=1474 pq=1 pqc=1473 qp=0 dt=1572/0/0 df=8 of=0 ri=1 ql=0 qs=.... kt=0/W/7 ktl=cf15 b=10 ci=0 co=0 ca=0

The first section lists the rcu_data structures for rcu_sched, the second
for rcu_bh.  Note that CONFIG_TREE_PREEMPT_RCU kernels will have an
additional section for rcu_preempt.  Each section has one line per CPU,
or eight for this 8-CPU system.  The fields are as follows:

o       The number at the beginning of each line is the CPU number.
        CPUs numbers followed by an exclamation mark are offline,
        but have been online at least once since boot.  There will be
        no output for CPUs that have never been online, which can be
        a good thing in the surprisingly common case where NR_CPUS is
        substantially larger than the number of actual CPUs.

o       "c" is the count of grace periods that this CPU believes have
        completed.  Offlined CPUs and CPUs in dynticks idle mode may
        lag quite a ways behind, for example, CPU 6 under "rcu_sched"
        above, which has been offline through not quite 40,000 RCU grace
        periods.  It is not unusual to see CPUs lagging by thousands of
        grace periods.

o       "g" is the count of grace periods that this CPU believes have
        started.  Again, offlined CPUs and CPUs in dynticks idle mode
        may lag behind.  If the "c" and "g" values are equal, this CPU
        has already reported a quiescent state for the last RCU grace
        period that it is aware of, otherwise, the CPU believes that it
        owes RCU a quiescent state.

o       "pq" indicates that this CPU has passed through a quiescent state
        for the current grace period.  It is possible for "pq" to be
        "1" and "c" different than "g", which indicates that although
        the CPU has passed through a quiescent state, either (1) this
        CPU has not yet reported that fact, (2) some other CPU has not
        yet reported for this grace period, or (3) both.

o       "pqc" indicates which grace period the last-observed quiescent
        state for this CPU corresponds to.  This is important for handling
        the race between CPU 0 reporting an extended dynticks-idle
        quiescent state for CPU 1 and CPU 1 suddenly waking up and
        reporting its own quiescent state.  If CPU 1 was the last CPU
        for the current grace period, then the CPU that loses this race
        will attempt to incorrectly mark CPU 1 as having checked in for
        the next grace period!

o       "qp" indicates that RCU still expects a quiescent state from
        this CPU.  Offlined CPUs and CPUs in dyntick idle mode might
        well have qp=1, which is OK: RCU is still ignoring them.

o       "dt" is the current value of the dyntick counter that is incremented
        when entering or leaving dynticks idle state, either by the
        scheduler or by irq.  This number is even if the CPU is in
        dyntick idle mode and odd otherwise.  The number after the first
        "/" is the interrupt nesting depth when in dyntick-idle state,
        or one greater than the interrupt-nesting depth otherwise.
        The number after the second "/" is the NMI nesting depth.

        This field is displayed only for CONFIG_NO_HZ kernels.

o       "df" is the number of times that some other CPU has forced a
        quiescent state on behalf of this CPU due to this CPU being in
        dynticks-idle state.

        This field is displayed only for CONFIG_NO_HZ kernels.

o       "of" is the number of times that some other CPU has forced a
        quiescent state on behalf of this CPU due to this CPU being
        offline.  In a perfect world, this might never happen, but it
        turns out that offlining and onlining a CPU can take several grace
        periods, and so there is likely to be an extended period of time
        when RCU believes that the CPU is online when it really is not.
        Please note that erring in the other direction (RCU believing a
        CPU is offline when it is really alive and kicking) is a fatal
        error, so it makes sense to err conservatively.

o       "ri" is the number of times that RCU has seen fit to send a
        reschedule IPI to this CPU in order to get it to report a
        quiescent state.

o       "ql" is the number of RCU callbacks currently residing on
        this CPU.  This is the total number of callbacks, regardless
        of what state they are in (new, waiting for grace period to
        start, waiting for grace period to end, ready to invoke).

o       "qs" gives an indication of the state of the callback queue
        with four characters:

        "N"     Indicates that there are callbacks queued that are not
                ready to be handled by the next grace period, and thus
                will be handled by the grace period following the next
                one.

        "R"     Indicates that there are callbacks queued that are
                ready to be handled by the next grace period.

        "W"     Indicates that there are callbacks queued that are
                waiting on the current grace period.

        "D"     Indicates that there are callbacks queued that have
                already been handled by a prior grace period, and are
                thus waiting to be invoked.  Note that callbacks in
                the process of being invoked are not counted here.
                Callbacks in the process of being invoked are those
                that have been removed from the rcu_data structures
                queues by rcu_do_batch(), but which have not yet been
                invoked.

        If there are no callbacks in a given one of the above states,
        the corresponding character is replaced by ".".

o       "kt" is the per-CPU kernel-thread state.  The digit preceding
        the first slash is zero if there is no work pending and 1
        otherwise.  The character between the first pair of slashes is
        as follows:

        "S"     The kernel thread is stopped, in other words, all
                CPUs corresponding to this rcu_node structure are
                offline.

        "R"     The kernel thread is running.

        "W"     The kernel thread is waiting because there is no work
                for it to do.

        "O"     The kernel thread is waiting because it has been
                forced off of its designated CPU or because its
                ->cpus_allowed mask permits it to run on other than
                its designated CPU.

        "Y"     The kernel thread is yielding to avoid hogging CPU.

        "?"     Unknown value, indicates a bug.

        The number after the final slash is the CPU that the kthread
        is actually running on.

        This field is displayed only for CONFIG_RCU_BOOST kernels.

o       "ktl" is the low-order 16 bits (in hexadecimal) of the count of
        the number of times that this CPU's per-CPU kthread has gone
        through its loop servicing invoke_rcu_cpu_kthread() requests.

        This field is displayed only for CONFIG_RCU_BOOST kernels.

o       "b" is the batch limit for this CPU.  If more than this number
        of RCU callbacks is ready to invoke, then the remainder will
        be deferred.

o       "ci" is the number of RCU callbacks that have been invoked for
        this CPU.  Note that ci+ql is the number of callbacks that have
        been registered in absence of CPU-hotplug activity.

o       "co" is the number of RCU callbacks that have been orphaned due to
        this CPU going offline.  These orphaned callbacks have been moved
        to an arbitrarily chosen online CPU.

o       "ca" is the number of RCU callbacks that have been adopted due to
        other CPUs going offline.  Note that ci+co-ca+ql is the number of
        RCU callbacks registered on this CPU.

There is also an rcu/rcudata.csv file with the same information in
comma-separated-variable spreadsheet format.


The output of "cat rcu/rcugp" looks as follows:

rcu_sched: completed=33062  gpnum=33063
rcu_bh: completed=464  gpnum=464

Again, this output is for both "rcu_sched" and "rcu_bh".  Note that
kernels built with CONFIG_TREE_PREEMPT_RCU will have an additional
"rcu_preempt" line.  The fields are taken from the rcu_state structure,
and are as follows:

o       "completed" is the number of grace periods that have completed.
        It is comparable to the "c" field from rcu/rcudata in that a
        CPU whose "c" field matches the value of "completed" is aware
        that the corresponding RCU grace period has completed.

o       "gpnum" is the number of grace periods that have started.  It is
        comparable to the "g" field from rcu/rcudata in that a CPU
        whose "g" field matches the value of "gpnum" is aware that the
        corresponding RCU grace period has started.

        If these two fields are equal (as they are for "rcu_bh" above),
        then there is no grace period in progress, in other words, RCU
        is idle.  On the other hand, if the two fields differ (as they
        do for "rcu_sched" above), then an RCU grace period is in progress.


The output of "cat rcu/rcuhier" looks as follows, with very long lines:

c=6902 g=6903 s=2 jfq=3 j=72c7 nfqs=13142/nfqsng=0(13142) fqlh=6
1/1 ..>. 0:127 ^0
3/3 ..>. 0:35 ^0    0/0 ..>. 36:71 ^1    0/0 ..>. 72:107 ^2    0/0 ..>. 108:127 ^3
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
rcu_bh:
c=-226 g=-226 s=1 jfq=-5701 j=72c7 nfqs=88/nfqsng=0(88) fqlh=0
0/1 ..>. 0:127 ^0
0/3 ..>. 0:35 ^0    0/0 ..>. 36:71 ^1    0/0 ..>. 72:107 ^2    0/0 ..>. 108:127 ^3
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

This is once again split into "rcu_sched" and "rcu_bh" portions,
and CONFIG_TREE_PREEMPT_RCU kernels will again have an additional
"rcu_preempt" section.  The fields are as follows:

o       "c" is exactly the same as "completed" under rcu/rcugp.

o       "g" is exactly the same as "gpnum" under rcu/rcugp.

o       "s" is the "signaled" state that drives force_quiescent_state()'s
        state machine.

o       "jfq" is the number of jiffies remaining for this grace period
        before force_quiescent_state() is invoked to help push things
        along.  Note that CPUs in dyntick-idle mode throughout the grace
        period will not report on their own, but rather must be check by
        some other CPU via force_quiescent_state().

o       "j" is the low-order four hex digits of the jiffies counter.
        Yes, Paul did run into a number of problems that turned out to
        be due to the jiffies counter no longer counting.  Why do you ask?

o       "nfqs" is the number of calls to force_quiescent_state() since
        boot.

o       "nfqsng" is the number of useless calls to force_quiescent_state(),
        where there wasn't actually a grace period active.  This can
        happen due to races.  The number in parentheses is the difference
        between "nfqs" and "nfqsng", or the number of times that
        force_quiescent_state() actually did some real work.

o       "fqlh" is the number of calls to force_quiescent_state() that
        exited immediately (without even being counted in nfqs above)
        due to contention on ->fqslock.

o       Each element of the form "1/1 0:127 ^0" represents one struct
        rcu_node.  Each line represents one level of the hierarchy, from
        root to leaves.  It is best to think of the rcu_data structures
        as forming yet another level after the leaves.  Note that there
        might be either one, two, or three levels of rcu_node structures,
        depending on the relationship between CONFIG_RCU_FANOUT and
        CONFIG_NR_CPUS.

        o       The numbers separated by the "/" are the qsmask followed
                by the qsmaskinit.  The qsmask will have one bit
                set for each entity in the next lower level that
                has not yet checked in for the current grace period.
                The qsmaskinit will have one bit for each entity that is
                currently expected to check in during each grace period.
                The value of qsmaskinit is assigned to that of qsmask
                at the beginning of each grace period.

                For example, for "rcu_sched", the qsmask of the first
                entry of the lowest level is 0x14, meaning that we
                are still waiting for CPUs 2 and 4 to check in for the
                current grace period.

        o       The characters separated by the ">" indicate the state
                of the blocked-tasks lists.  A "G" preceding the ">"
                indicates that at least one task blocked in an RCU
                read-side critical section blocks the current grace
                period, while a "E" preceding the ">" indicates that
                at least one task blocked in an RCU read-side critical
                section blocks the current expedited grace period.
                A "T" character following the ">" indicates that at
                least one task is blocked within an RCU read-side
                critical section, regardless of whether any current
                grace period (expedited or normal) is inconvenienced.
                A "." character appears if the corresponding condition
                does not hold, so that "..>." indicates that no tasks
                are blocked.  In contrast, "GE>T" indicates maximal
                inconvenience from blocked tasks.

        o       The numbers separated by the ":" are the range of CPUs
                served by this struct rcu_node.  This can be helpful
                in working out how the hierarchy is wired together.

                For example, the first entry at the lowest level shows
                "0:5", indicating that it covers CPUs 0 through 5.

        o       The number after the "^" indicates the bit in the
                next higher level rcu_node structure that this
                rcu_node structure corresponds to.

                For example, the first entry at the lowest level shows
                "^0", indicating that it corresponds to bit zero in
                the first entry at the middle level.


The output of "cat rcu/rcu_pending" looks as follows:

rcu_sched:
  0 np=255892 qsp=53936 rpq=85 cbr=0 cng=14417 gpc=10033 gps=24320 nf=6445 nn=146741
  1 np=261224 qsp=54638 rpq=33 cbr=0 cng=25723 gpc=16310 gps=2849 nf=5912 nn=155792
  2 np=237496 qsp=49664 rpq=23 cbr=0 cng=2762 gpc=45478 gps=1762 nf=1201 nn=136629
  3 np=236249 qsp=48766 rpq=98 cbr=0 cng=286 gpc=48049 gps=1218 nf=207 nn=137723
  4 np=221310 qsp=46850 rpq=7 cbr=0 cng=26 gpc=43161 gps=4634 nf=3529 nn=123110
  5 np=237332 qsp=48449 rpq=9 cbr=0 cng=54 gpc=47920 gps=3252 nf=201 nn=137456
  6 np=219995 qsp=46718 rpq=12 cbr=0 cng=50 gpc=42098 gps=6093 nf=4202 nn=120834
  7 np=249893 qsp=49390 rpq=42 cbr=0 cng=72 gpc=38400 gps=17102 nf=41 nn=144888
rcu_bh:
  0 np=146741 qsp=1419 rpq=6 cbr=0 cng=6 gpc=0 gps=0 nf=2 nn=145314
  1 np=155792 qsp=12597 rpq=3 cbr=0 cng=0 gpc=4 gps=8 nf=3 nn=143180
  2 np=136629 qsp=18680 rpq=1 cbr=0 cng=0 gpc=7 gps=6 nf=0 nn=117936
  3 np=137723 qsp=2843 rpq=0 cbr=0 cng=0 gpc=10 gps=7 nf=0 nn=134863
  4 np=123110 qsp=12433 rpq=0 cbr=0 cng=0 gpc=4 gps=2 nf=0 nn=110671
  5 np=137456 qsp=4210 rpq=1 cbr=0 cng=0 gpc=6 gps=5 nf=0 nn=133235
  6 np=120834 qsp=9902 rpq=2 cbr=0 cng=0 gpc=6 gps=3 nf=2 nn=110921
  7 np=144888 qsp=26336 rpq=0 cbr=0 cng=0 gpc=8 gps=2 nf=0 nn=118542

As always, this is once again split into "rcu_sched" and "rcu_bh"
portions, with CONFIG_TREE_PREEMPT_RCU kernels having an additional
"rcu_preempt" section.  The fields are as follows:

o       "np" is the number of times that __rcu_pending() has been invoked
        for the corresponding flavor of RCU.

o       "qsp" is the number of times that the RCU was waiting for a
        quiescent state from this CPU.

o       "rpq" is the number of times that the CPU had passed through
        a quiescent state, but not yet reported it to RCU.

o       "cbr" is the number of times that this CPU had RCU callbacks
        that had passed through a grace period, and were thus ready
        to be invoked.

o       "cng" is the number of times that this CPU needed another
        grace period while RCU was idle.

o       "gpc" is the number of times that an old grace period had
        completed, but this CPU was not yet aware of it.

o       "gps" is the number of times that a new grace period had started,
        but this CPU was not yet aware of it.

o       "nf" is the number of times that this CPU suspected that the
        current grace period had run for too long, and thus needed to
        be forced.

        Please note that "forcing" consists of sending resched IPIs
        to holdout CPUs.  If that CPU really still is in an old RCU
        read-side critical section, then we really do have to wait for it.
        The assumption behing "forcing" is that the CPU is not still in
        an old RCU read-side critical section, but has not yet responded
        for some other reason.

o       "nn" is the number of times that this CPU needed nothing.  Alert
        readers will note that the rcu "nn" number for a given CPU very
        closely matches the rcu_bh "np" number for that same CPU.  This
        is due to short-circuit evaluation in rcu_pending().


The output of "cat rcu/rcutorture" looks as follows:

rcutorture test sequence: 0 (test in progress)
rcutorture update version number: 615

The first line shows the number of rcutorture tests that have completed
since boot.  If a test is currently running, the "(test in progress)"
string will appear as shown above.  The second line shows the number of
update cycles that the current test has started, or zero if there is
no test in progress.


The output of "cat rcu/rcuboost" looks as follows:

0:5 tasks=.... kt=W ntb=0 neb=0 nnb=0 j=2f95 bt=300f
     balk: nt=0 egt=989 bt=0 nb=0 ny=0 nos=16
6:7 tasks=.... kt=W ntb=0 neb=0 nnb=0 j=2f95 bt=300f
     balk: nt=0 egt=225 bt=0 nb=0 ny=0 nos=6

This information is output only for rcu_preempt.  Each two-line entry
corresponds to a leaf rcu_node strcuture.  The fields are as follows:

o       "n:m" is the CPU-number range for the corresponding two-line
        entry.  In the sample output above, the first entry covers
        CPUs zero through five and the second entry covers CPUs 6
        and 7.

o       "tasks=TNEB" gives the state of the various segments of the
        rnp->blocked_tasks list:

        "T"     This indicates that there are some tasks that blocked
                while running on one of the corresponding CPUs while
                in an RCU read-side critical section.

        "N"     This indicates that some of the blocked tasks are preventing
                the current normal (non-expedited) grace period from
                completing.

        "E"     This indicates that some of the blocked tasks are preventing
                the current expedited grace period from completing.

        "B"     This indicates that some of the blocked tasks are in
                need of RCU priority boosting.

        Each character is replaced with "." if the corresponding
        condition does not hold.

o       "kt" is the state of the RCU priority-boosting kernel
        thread associated with the corresponding rcu_node structure.
        The state can be one of the following:

        "S"     The kernel thread is stopped, in other words, all
                CPUs corresponding to this rcu_node structure are
                offline.

        "R"     The kernel thread is running.

        "W"     The kernel thread is waiting because there is no work
                for it to do.

        "Y"     The kernel thread is yielding to avoid hogging CPU.

        "?"     Unknown value, indicates a bug.

o       "ntb" is the number of tasks boosted.

o       "neb" is the number of tasks boosted in order to complete an
        expedited grace period.

o       "nnb" is the number of tasks boosted in order to complete a
        normal (non-expedited) grace period.  When boosting a task
        that was blocking both an expedited and a normal grace period,
        it is counted against the expedited total above.

o       "j" is the low-order 16 bits of the jiffies counter in
        hexadecimal.

o       "bt" is the low-order 16 bits of the value that the jiffies
        counter will have when we next start boosting, assuming that
        the current grace period does not end beforehand.  This is
        also in hexadecimal.

o       "balk: nt" counts the number of times we didn't boost (in
        other words, we balked) even though it was time to boost because
        there were no blocked tasks to boost.  This situation occurs
        when there is one blocked task on one rcu_node structure and
        none on some other rcu_node structure.

o       "egt" counts the number of times we balked because although
        there were blocked tasks, none of them were blocking the
        current grace period, whether expedited or otherwise.

o       "bt" counts the number of times we balked because boosting
        had already been initiated for the current grace period.

o       "nb" counts the number of times we balked because there
        was at least one task blocking the current non-expedited grace
        period that never had blocked.  If it is already running, it
        just won't help to boost its priority!

o       "ny" counts the number of times we balked because it was
        not yet time to start boosting.

o       "nos" counts the number of times we balked for other
        reasons, e.g., the grace period ended first.


CONFIG_TINY_RCU and CONFIG_TINY_PREEMPT_RCU debugfs Files and Formats

These implementations of RCU provides a single debugfs file under the
top-level directory RCU, namely rcu/rcudata, which displays fields in
rcu_bh_ctrlblk, rcu_sched_ctrlblk and, for CONFIG_TINY_PREEMPT_RCU,
rcu_preempt_ctrlblk.

The output of "cat rcu/rcudata" is as follows:

rcu_preempt: qlen=24 gp=1097669 g197/p197/c197 tasks=...
             ttb=. btg=no ntb=184 neb=0 nnb=183 j=01f7 bt=0274
             normal balk: nt=1097669 gt=0 bt=371 b=0 ny=25073378 nos=0
             exp balk: bt=0 nos=0
rcu_sched: qlen: 0
rcu_bh: qlen: 0

This is split into rcu_preempt, rcu_sched, and rcu_bh sections, with the
rcu_preempt section appearing only in CONFIG_TINY_PREEMPT_RCU builds.
The last three lines of the rcu_preempt section appear only in
CONFIG_RCU_BOOST kernel builds.  The fields are as follows:

o       "qlen" is the number of RCU callbacks currently waiting either
        for an RCU grace period or waiting to be invoked.  This is the
        only field present for rcu_sched and rcu_bh, due to the
        short-circuiting of grace period in those two cases.

o       "gp" is the number of grace periods that have completed.

o       "g197/p197/c197" displays the grace-period state, with the
        "g" number being the number of grace periods that have started
        (mod 256), the "p" number being the number of grace periods
        that the CPU has responded to (also mod 256), and the "c"
        number being the number of grace periods that have completed
        (once again mode 256).

        Why have both "gp" and "g"?  Because the data flowing into
        "gp" is only present in a CONFIG_RCU_TRACE kernel.

o       "tasks" is a set of bits.  The first bit is "T" if there are
        currently tasks that have recently blocked within an RCU
        read-side critical section, the second bit is "N" if any of the
        aforementioned tasks are blocking the current RCU grace period,
        and the third bit is "E" if any of the aforementioned tasks are
        blocking the current expedited grace period.  Each bit is "."
        if the corresponding condition does not hold.

o       "ttb" is a single bit.  It is "B" if any of the blocked tasks
        need to be priority boosted and "." otherwise.

o       "btg" indicates whether boosting has been carried out during
        the current grace period, with "exp" indicating that boosting
        is in progress for an expedited grace period, "no" indicating
        that boosting has not yet started for a normal grace period,
        "begun" indicating that boosting has bebug for a normal grace
        period, and "done" indicating that boosting has completed for
        a normal grace period.

o       "ntb" is the total number of tasks subjected to RCU priority boosting
        periods since boot.

o       "neb" is the number of expedited grace periods that have had
        to resort to RCU priority boosting since boot.

o       "nnb" is the number of normal grace periods that have had
        to resort to RCU priority boosting since boot.

o       "j" is the low-order 16 bits of the jiffies counter in hexadecimal.

o       "bt" is the low-order 16 bits of the value that the jiffies counter
        will have at the next time that boosting is scheduled to begin.

o       In the line beginning with "normal balk", the fields are as follows:

        o       "nt" is the number of times that the system balked from
                boosting because there were no blocked tasks to boost.
                Note that the system will balk from boosting even if the
                grace period is overdue when the currently running task
                is looping within an RCU read-side critical section.
                There is no point in boosting in this case, because
                boosting a running task won't make it run any faster.

        o       "gt" is the number of times that the system balked
                from boosting because, although there were blocked tasks,
                none of them were preventing the current grace period
                from completing.

        o       "bt" is the number of times that the system balked
                from boosting because boosting was already in progress.

        o       "b" is the number of times that the system balked from
                boosting because boosting had already completed for
                the grace period in question.

        o       "ny" is the number of times that the system balked from
                boosting because it was not yet time to start boosting
                the grace period in question.

        o       "nos" is the number of times that the system balked from
                boosting for inexplicable ("not otherwise specified")
                reasons.  This can actually happen due to races involving
                increments of the jiffies counter.

o       In the line beginning with "exp balk", the fields are as follows:

        o       "bt" is the number of times that the system balked from
                boosting because there were no blocked tasks to boost.

        o       "nos" is the number of times that the system balked from
                 boosting for inexplicable ("not otherwise specified")
                 reasons.