2036d94a7b
Because newly offlined CPUs continue executing after completing the CPU_DYING notifiers, they legitimately enter the scheduler and use RCU while appearing to be offline. This calls for a more sophisticated approach as follows: 1. RCU marks the CPU online during the CPU_UP_PREPARE phase. 2. RCU marks the CPU offline during the CPU_DEAD phase. 3. Diagnostics regarding use of read-side RCU by offline CPUs use RCU's accounting rather than the cpu_online_map. (Note that __call_rcu() still uses cpu_online_map to detect illegal invocations within CPU_DYING notifiers.) 4. Offline CPUs are prevented from hanging the system by force_quiescent_state(), which pays attention to cpu_online_map. Some additional work (in a later commit) will be needed to guarantee that force_quiescent_state() waits a full jiffy before assuming that a CPU is offline, for example, when called from idle entry. (This commit also makes the one-jiffy wait explicit, since the old-style implicit wait can now be defeated by RCU_FAST_NO_HZ and by rcutorture.) This approach avoids the false positives encountered when attempting to use more exact classification of CPU online/offline state. Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
614 lines
27 KiB
Plaintext
614 lines
27 KiB
Plaintext
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 pgp=20973 qp=0 dt=545/1/0 df=50 of=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 pgp=20973 qp=0 dt=967/1/0 df=58 of=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 pgp=20973 qp=0 dt=1081/1/0 df=175 of=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 pgp=20942 qp=1 dt=1846/0/0 df=404 of=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 pgp=20973 qp=0 dt=369/1/0 df=83 of=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 pgp=20973 qp=0 dt=381/1/0 df=64 of=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 pgp=20973 qp=0 dt=1037/1/0 df=183 of=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 pgp=20896 qp=0 dt=1572/0/0 df=382 of=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 pgp=1480 qp=0 dt=545/1/0 df=6 of=0 ql=0 qs=.... kt=0/W/0 ktl=ebc3 b=10 ci=0 co=0 ca=0
|
|
1 c=1480 g=1480 pq=1 pgp=1480 qp=0 dt=967/1/0 df=3 of=0 ql=0 qs=.... kt=0/W/1 ktl=58c b=10 ci=151 co=0 ca=0
|
|
2 c=1480 g=1480 pq=1 pgp=1480 qp=0 dt=1081/1/0 df=6 of=0 ql=0 qs=.... kt=0/W/2 ktl=da94 b=10 ci=0 co=0 ca=0
|
|
3 c=1480 g=1480 pq=1 pgp=1480 qp=0 dt=1846/0/0 df=8 of=0 ql=0 qs=.... kt=0/W/3 ktl=d1cd b=10 ci=0 co=0 ca=0
|
|
4 c=1480 g=1480 pq=1 pgp=1480 qp=0 dt=369/1/0 df=6 of=0 ql=0 qs=.... kt=0/W/4 ktl=e0e7 b=10 ci=0 co=0 ca=0
|
|
5 c=1480 g=1480 pq=1 pgp=1480 qp=0 dt=381/1/0 df=4 of=0 ql=0 qs=.... kt=0/W/5 ktl=fb2f b=10 ci=0 co=0 ca=0
|
|
6 c=1480 g=1480 pq=1 pgp=1480 qp=0 dt=1037/1/0 df=6 of=0 ql=0 qs=.... kt=0/W/6 ktl=d2ad b=10 ci=0 co=0 ca=0
|
|
7 c=1474 g=1474 pq=1 pgp=1473 qp=0 dt=1572/0/0 df=8 of=0 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 "pgp" 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.
|
|
|
|
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.
|
|
|
|
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 "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.
|