[PATCH tip/core/rcu 1/6] rcu: Update trace.txt documentation to reflect recent changes

Paul E. McKenney
2009-10-14T17:29:14Z

o Remove the CONFIG_PREEMPT_RCU documentation since this
config option has now been removed.

o Change the now-incorrect references to "rcu" labels to
instead be "rcu_sched".

o Add notes stating that CONFIG_TREE_PREEMPT_RCU kernels will
have additional "rcu_preempt" output.

o Note the new "oqlen" field in the rcuhier output (for
RCU callbacks orphaned by an offlined CPU).

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
Documentation/RCU/trace.txt | 231 ++++++------------------------------------
1 files changed, 33 insertions(+), 198 deletions(-)

diff --git a/Documentation/RCU/trace.txt b/Documentation/RCU/trace.txt
index 187bbf1..c1a9550 100644
--- a/Documentation/RCU/trace.txt
+++ b/Documentation/RCU/trace.txt
@@ -1,185 +1,10 @@
CONFIG_RCU_TRACE debugfs Files and Formats

-The rcupreempt and rcutree 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.
-Note that the rcuclassic implementation of RCU does not provide debugfs
-trace output.
-
-The following sections describe the debugfs files and formats for
-preemptable RCU (rcupreempt) and hierarchical RCU (rcutree).
-
-
-Preemptable RCU debugfs Files and Formats
-
-This implementation of RCU provides three debugfs files under the
-top-level directory RCU: rcu/rcuctrs (which displays the per-CPU
-counters used by preemptable RCU) rcu/rcugp (which displays grace-period
-counters), and rcu/rcustats (which internal counters for debugging RCU).
-
-The output of "cat rcu/rcuctrs" looks as follows:
-
-CPU last cur F M
- 0 5 -5 0 0
- 1 -1 0 0 0
- 2 0 1 0 0
- 3 0 1 0 0
- 4 0 1 0 0
- 5 0 1 0 0
- 6 0 2 0 0
- 7 0 -1 0 0
- 8 0 1 0 0
-ggp = 26226, state = waitzero
-
-The per-CPU fields are as follows:
-
-o "CPU" gives the CPU number. Offline CPUs are not displayed.
-
-o "last" gives the value of the counter that is being decremented
- for the current grace period phase. In the example above,
- the counters sum to 4, indicating that there are still four
- RCU read-side critical sections still running that started
- before the last counter flip.
-
-o "cur" gives the value of the counter that is currently being
- both incremented (by rcu_read_lock()) and decremented (by
- rcu_read_unlock()). In the example above, the counters sum to
- 1, indicating that there is only one RCU read-side critical section
- still running that started after the last counter flip.
-
-o "F" indicates whether RCU is waiting for this CPU to acknowledge
- a counter flip. In the above example, RCU is not waiting on any,
- which is consistent with the state being "waitzero" rather than
- "waitack".
-
-o "M" indicates whether RCU is waiting for this CPU to execute a
- memory barrier. In the above example, RCU is not waiting on any,
- which is consistent with the state being "waitzero" rather than
- "waitmb".
-
-o "ggp" is the global grace-period counter.
-
-o "state" is the RCU state, which can be one of the following:
-
- o "idle": there is no grace period in progress.
-
- o "waitack": RCU just incremented the global grace-period
- counter, which has the effect of reversing the roles of
- the "last" and "cur" counters above, and is waiting for
- all the CPUs to acknowledge the flip. Once the flip has
- been acknowledged, CPUs will no longer be incrementing
- what are now the "last" counters, so that their sum will
- decrease monotonically down to zero.
-
- o "waitzero": RCU is waiting for the sum of the "last" counters
- to decrease to zero.
-
- o "waitmb": RCU is waiting for each CPU to execute a memory
- barrier, which ensures that instructions from a given CPU's
- last RCU read-side critical section cannot be reordered
- with instructions following the memory-barrier instruction.
-
-The output of "cat rcu/rcugp" looks as follows:
-
-oldggp=48870 newggp=48873
-
-Note that reading from this file provokes a synchronize_rcu(). The
-"oldggp" value is that of "ggp" from rcu/rcuctrs above, taken before
-executing the synchronize_rcu(), and the "newggp" value is also the
-"ggp" value, but taken after the synchronize_rcu() command returns.
-
-
-The output of "cat rcu/rcugp" looks as follows:
-
-na=1337955 nl=40 wa=1337915 wl=44 da=1337871 dl=0 dr=1337871 di=1337871
-1=50989 e1=6138 i1=49722 ie1=82 g1=49640 a1=315203 ae1=265563 a2=49640
-z1=1401244 ze1=1351605 z2=49639 m1=5661253 me1=5611614 m2=49639
-
-These are counters tracking internal preemptable-RCU events, however,
-some of them may be useful for debugging algorithms using RCU. In
-particular, the "nl", "wl", and "dl" values track the number of RCU
-callbacks in various states. The fields are as follows:
-
-o "na" is the total number of RCU callbacks that have been enqueued
- since boot.
-
-o "nl" is the number of RCU callbacks waiting for the previous
- grace period to end so that they can start waiting on the next
- grace period.
-
-o "wa" is the total number of RCU callbacks that have started waiting
- for a grace period since boot. "na" should be roughly equal to
- "nl" plus "wa".
-
-o "wl" is the number of RCU callbacks currently waiting for their
- grace period to end.
-
-o "da" is the total number of RCU callbacks whose grace periods
- have completed since boot. "wa" should be roughly equal to
- "wl" plus "da".
-
-o "dr" is the total number of RCU callbacks that have been removed
- from the list of callbacks ready to invoke. "dr" should be roughly
- equal to "da".
-
-o "di" is the total number of RCU callbacks that have been invoked
- since boot. "di" should be roughly equal to "da", though some
- early versions of preemptable RCU had a bug so that only the
- last CPU's count of invocations was displayed, rather than the
- sum of all CPU's counts.
-
-o "1" is the number of calls to rcu_try_flip(). This should be
- roughly equal to the sum of "e1", "i1", "a1", "z1", and "m1"
- described below. In other words, the number of times that
- the state machine is visited should be equal to the sum of the
- number of times that each state is visited plus the number of
- times that the state-machine lock acquisition failed.
-
-o "e1" is the number of times that rcu_try_flip() was unable to
- acquire the fliplock.
-
-o "i1" is the number of calls to rcu_try_flip_idle().
-
-o "ie1" is the number of times rcu_try_flip_idle() exited early
- due to the calling CPU having no work for RCU.
-
-o "g1" is the number of times that rcu_try_flip_idle() decided
- to start a new grace period. "i1" should be roughly equal to
- "ie1" plus "g1".
-
-o "a1" is the number of calls to rcu_try_flip_waitack().
-
-o "ae1" is the number of times that rcu_try_flip_waitack() found
- that at least one CPU had not yet acknowledge the new grace period
- (AKA "counter flip").
-
-o "a2" is the number of time rcu_try_flip_waitack() found that
- all CPUs had acknowledged. "a1" should be roughly equal to
- "ae1" plus "a2". (This particular output was collected on
- a 128-CPU machine, hence the smaller-than-usual fraction of
- calls to rcu_try_flip_waitack() finding all CPUs having already
- acknowledged.)
-
-o "z1" is the number of calls to rcu_try_flip_waitzero().
-
-o "ze1" is the number of times that rcu_try_flip_waitzero() found
- that not all of the old RCU read-side critical sections had
- completed.
-
-o "z2" is the number of times that rcu_try_flip_waitzero() finds
- the sum of the counters equal to zero, in other words, that
- all of the old RCU read-side critical sections had completed.
- The value of "z1" should be roughly equal to "ze1" plus
- "z2".
-
-o "m1" is the number of calls to rcu_try_flip_waitmb().
-
-o "me1" is the number of times that rcu_try_flip_waitmb() finds
- that at least one CPU has not yet executed a memory barrier.
-
-o "m2" is the number of times that rcu_try_flip_waitmb() finds that
- all CPUs have executed a memory barrier.
+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
@@ -210,9 +35,10 @@ rcu_bh:
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, the second for
-rcu_bh. Each section has one line per CPU, or eight for this 8-CPU system.
-The fields are as follows:
+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,
@@ -223,9 +49,9 @@ o The number at the beginning of each line is the CPU number.

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" above, which has slept
- through the past 25 RCU grace periods. It is not unusual to
- see CPUs lagging by thousands of grace periods.
+ 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.
@@ -308,8 +134,10 @@ 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" and "rcu_bh". The fields are
-taken from the rcu_state structure, and are as follows:
+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
@@ -324,23 +152,24 @@ o "gpnum" is the number of grace periods that have started. It is
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" above), then an RCU grace period is in progress.
+ 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
+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
+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" and "rcu_bh" portions. The fields are
-as follows:
+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.

@@ -372,6 +201,11 @@ 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
@@ -389,10 +223,10 @@ o Each element of the form "1/1 0:127 ^0" represents one struct
The value of qsmaskinit is assigned to that of qsmask
at the beginning of each grace period.

- For example, for "rcu", 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.
+ 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 numbers separated by the ":" are the range of CPUs
served by this struct rcu_node. This can be helpful
@@ -431,8 +265,9 @@ rcu_bh:
6 np=120834 qsp=9902 cbr=0 cng=0 gpc=6 gps=3 nf=2 nn=110921
7 np=144888 qsp=26336 cbr=0 cng=0 gpc=8 gps=2 nf=0 nn=118542

-As always, this is once again split into "rcu" and "rcu_bh" portions.
-The fields are as follows:
+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.
--
1.5.2.5