mirror of
https://mirrors.bfsu.edu.cn/git/linux.git
synced 2024-11-17 09:14:19 +08:00
d21670acab
Lai Jiangshan noted that up to 10% of the RCU_SOFTIRQ are spurious, and traced this down to the fact that the current grace-period machinery will uselessly raise RCU_SOFTIRQ when a given CPU needs to go through a quiescent state, but has not yet done so. In this situation, there might well be nothing that RCU_SOFTIRQ can do, and the overhead can be worth worrying about in the ksoftirqd case. This patch therefore avoids raising RCU_SOFTIRQ in this situation. Changes since v1 (http://lkml.org/lkml/2010/3/30/122 from Lai Jiangshan): o Omit the rcu_qs_pending() prechecks, as they aren't that much less expensive than the quiescent-state checks. o Merge with the set_need_resched() patch that reduces IPIs. o Add the new n_rp_report_qs field to the rcu_pending tracing output. o Update the tracing documentation accordingly. Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
318 lines
15 KiB
Plaintext
318 lines
15 KiB
Plaintext
CONFIG_RCU_TRACE debugfs Files and Formats
|
|
|
|
|
|
The rcutree implementation of RCU provides 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.
|
|
|
|
|
|
Hierarchical RCU debugfs Files and Formats
|
|
|
|
This implementation of RCU provides three debugfs files under the
|
|
top-level directory RCU: rcu/rcudata (which displays fields in struct
|
|
rcu_data), rcu/rcugp (which displays grace-period counters), and
|
|
rcu/rcuhier (which displays the struct rcu_node hierarchy).
|
|
|
|
The output of "cat rcu/rcudata" looks as follows:
|
|
|
|
rcu_sched:
|
|
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
|
|
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
|
|
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
|
|
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
|
|
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
|
|
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
|
|
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
|
|
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
|
|
rcu_bh:
|
|
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
|
|
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
|
|
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
|
|
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
|
|
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
|
|
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
|
|
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
|
|
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
|
|
|
|
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. CPUs in dynticks idle mode may lag quite a ways
|
|
behind, for example, CPU 4 under "rcu_sched" above, which has
|
|
slept through the past 25 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, 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.
|
|
|
|
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. The number after the "/" is the interrupt
|
|
nesting depth when in dyntick-idle state, or one greater than
|
|
the interrupt-nesting depth otherwise.
|
|
|
|
This field is displayed only for CONFIG_NO_HZ kernels.
|
|
|
|
o "dn" is the current value of the dyntick counter that is incremented
|
|
when entering or leaving dynticks idle state via NMI. If both
|
|
the "dt" and "dn" values are even, then this CPU is in dynticks
|
|
idle mode and may be ignored by RCU. If either of these two
|
|
counters is odd, then RCU must be alert to the possibility of
|
|
an RCU read-side critical section running on this CPU.
|
|
|
|
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 neve 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 "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.
|
|
|
|
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 oqlen=0
|
|
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 oqlen=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 thoughout 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 "oqlen" is the number of callbacks on the "orphan" callback
|
|
list. RCU callbacks are placed on this list by CPUs going
|
|
offline, and are "adopted" either by the CPU helping the outgoing
|
|
CPU or by the next rcu_barrier*() call, whichever comes first.
|
|
|
|
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 "T" preceding the ">"
|
|
indicates that at least one task blocked in an RCU
|
|
read-side critical section blocks the current grace
|
|
period, while a "." preceding the ">" indicates otherwise.
|
|
The character following the ">" indicates similarly for
|
|
the next grace period. A "T" should appear in this
|
|
field only for rcu-preempt.
|
|
|
|
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().
|