Re: [PATCH tip/core/rcu 17/86] rcu: fix boost-tracing bug and update tracing documentation

Paul E. McKenney
2011-05-02T21:37:05Z

On Mon, May 02, 2011 at 01:33:01AM -0700, Paul E. McKenney wrote:
> On Sun, May 01, 2011 at 08:43:10AM -0700, Josh Triplett wrote:
> > The bugfix here should really go in a separate commit from the updated
> > documentation. Also:
> >
> > On Sun, May 01, 2011 at 06:20:57AM -0700, Paul E. McKenney wrote:
> > > --- a/Documentation/RCU/trace.txt
> > > +++ b/Documentation/RCU/trace.txt
> > > @@ -10,34 +10,37 @@ for rcutree and next for rcutiny.
> > >
> > > CONFIG_TREE_RCU and CONFIG_TREE_PREEMPT_RCU debugfs Files and Formats
> > >
> > > -These implementations of RCU provides five debugfs files under the
> > > +These implementations of RCU provides seven debugfs files under the
> >
> > s/seven/several/, so this doesn't continue to need updates as you add
> > more tracing? It doesn't seem entirely accurate even now, given that
> > the set of files available now depends on CONFIG_RCU_BOOST.
>
> Good point, will fix.
>
> > > top-level directory RCU: rcu/rcudata (which displays fields in struct
> >
> > RCU or rcu?
>
> Indeed, "rcu". Will fix.
>
> > > rcu_data), rcu/rcudata.csv (which is a .csv spreadsheet version of
> > > rcu/rcudata), rcu/rcugp (which displays grace-period counters),
> > > -rcu/rcuhier (which displays the struct rcu_node hierarchy), and
> > > +rcu/rcuhier (which displays the struct rcu_node hierarchy),
> > > rcu/rcu_pending (which displays counts of the reasons that the
> > > -rcu_pending() function decided that there was core RCU work to do).
> > > +rcu_pending() function decided that there was core RCU work to do),
> > > +rcu/rcutorture (which displays rcutorture test progress), and, if the
> > > +kernel is built with CONFIG_RCU_BOOST, rcu/rcuboost (which displays RCU
> > > +boosting statistics).
> >
> > I think this would work better as a list:
> >
> > 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.
> >
> > Feel free to put the descriptions on the same lines as the filenames if
> > you prefer; however, I thought this style worked better for the
> > descriptions longer than one line.
>
> Makes sense, will do a list.
>
> > Also, does rcu/rcutorture appear with rcutorture not loaded? Hopefully
> > not, since rcutorture shouldn't take up any additional resources when
> > not loaded.
>
> Not much choice, unfortunately. If I put the info into rcutorture, then
> I lose it when rcutorture is unloaded. This is problematic in automated
> tests that repeatedly load and unload rcutorture.
>
> But the amount of data is small, and I set things up so that TINY_RCU
> can omit this. TREE_RCU is big enough that it doesn't really matter.
>
> > > --- a/kernel/rcutree_plugin.h
> > > +++ b/kernel/rcutree_plugin.h
> > > @@ -1086,7 +1086,7 @@ static void rcu_initiate_boost_trace(struct rcu_node *rnp)
> > > else if (rnp->gp_tasks != NULL && rnp->qsmask != 0)
> > > rnp->n_balk_notblocked++;
> > > else if (rnp->gp_tasks != NULL &&
> > > - ULONG_CMP_GE(jiffies, rnp->boost_time))
> > > + ULONG_CMP_LT(jiffies, rnp->boost_time))
> > > rnp->n_balk_notyet++;
> > > else
> > > rnp->n_balk_nos++;
> >
> > This bugfix should go in a separate commit.
>
> Excellent point, will fix.

Here is the updated documentation update.

Thanx, Paul

------------------------------------------------------------------------

rcu: update tracing documentation for new rcutorture and rcuboost

This commit documents the new debugfs rcu/rcutorture and rcu/rcuboost
trace files. The description has been updated as suggested by Josh
Triplett.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --git a/Documentation/RCU/trace.txt b/Documentation/RCU/trace.txt
index 5aefd5f..40b530d 100644
--- a/Documentation/RCU/trace.txt
+++ b/Documentation/RCU/trace.txt
@@ -10,34 +10,46 @@ for rcutree and next for rcutiny.

CONFIG_TREE_RCU and CONFIG_TREE_PREEMPT_RCU debugfs Files and Formats

-These implementations of RCU provides five debugfs files under the
-top-level directory RCU: rcu/rcudata (which displays fields in struct
-rcu_data), rcu/rcudata.csv (which is a .csv spreadsheet version of
-rcu/rcudata), rcu/rcugp (which displays grace-period counters),
-rcu/rcuhier (which displays the struct rcu_node hierarchy), and
-rcu/rcu_pending (which displays counts of the reasons that the
-rcu_pending() function decided that there was core RCU work to do).
+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=423090 g=423091 pq=1 pqc=423090 qp=1 dt=86475/1/0 df=16319 of=163 ri=1519 ql=0 qs=.... b=10 ci=1460693 co=1648 ca=6448
- 1!c=423329 g=423330 pq=1 pqc=423329 qp=1 dt=90875/1/0 df=16231 of=157 ri=1249 ql=0 qs=.... b=10 ci=1459002 co=1614 ca=3310
- 2!c=423370 g=423371 pq=1 pqc=423370 qp=1 dt=69661/1/0 df=16125 of=163 ri=1469 ql=0 qs=.... b=10 ci=1610701 co=2015 ca=2378
- 3!c=422967 g=422968 pq=1 pqc=422967 qp=1 dt=70349/1/0 df=12528 of=163 ri=1450 ql=0 qs=.... b=10 ci=1427543 co=1430 ca=897
- 4!c=423196 g=423197 pq=1 pqc=423196 qp=0 dt=38935/1/0 df=10959 of=177 ri=1657 ql=0 qs=.... b=10 ci=1562249 co=1896 ca=533
- 5!c=422950 g=422951 pq=1 pqc=422950 qp=0 dt=25127/1/0 df=5895 of=167 ri=1549 ql=0 qs=.... b=10 ci=1777260 co=2137 ca=274
- 6!c=423396 g=423397 pq=1 pqc=423396 qp=1 dt=22639/1/0 df=4590 of=149 ri=1572 ql=0 qs=.... b=10 ci=1471186 co=1530 ca=243
- 7 c=460203 g=460203 pq=1 pqc=460202 qp=0 dt=937087/1/0 df=3298 of=149 ri=1584 ql=6 qs=N.W. b=10 ci=4026154 co=1948 ca=135
+ 0!c=423090 g=423091 pq=1 pqc=423090 qp=1 dt=86475/1/0 df=16319 of=163 ri=1519 ql=0 qs=.... kt=0/W b=10 ci=1460693 co=1648 ca=6448
+ 1!c=423329 g=423330 pq=1 pqc=423329 qp=1 dt=90875/1/0 df=16231 of=157 ri=1249 ql=0 qs=.... kt=0/W b=10 ci=1459002 co=1614 ca=3310
+ 2!c=423370 g=423371 pq=1 pqc=423370 qp=1 dt=69661/1/0 df=16125 of=163 ri=1469 ql=0 qs=.... kt=0/W b=10 ci=1610701 co=2015 ca=2378
+ 3!c=422967 g=422968 pq=1 pqc=422967 qp=1 dt=70349/1/0 df=12528 of=163 ri=1450 ql=0 qs=.... kt=0/W b=10 ci=1427543 co=1430 ca=897
+ 4!c=423196 g=423197 pq=1 pqc=423196 qp=0 dt=38935/1/0 df=10959 of=177 ri=1657 ql=0 qs=.... kt=0/W b=10 ci=1562249 co=1896 ca=533
+ 5!c=422950 g=422951 pq=1 pqc=422950 qp=0 dt=25127/1/0 df=5895 of=167 ri=1549 ql=0 qs=.... kt=0/W b=10 ci=1777260 co=2137 ca=274
+ 6!c=423396 g=423397 pq=1 pqc=423396 qp=1 dt=22639/1/0 df=4590 of=149 ri=1572 ql=0 qs=.... kt=0/W b=10 ci=1471186 co=1530 ca=243
+ 7 c=460203 g=460203 pq=1 pqc=460202 qp=0 dt=937087/1/0 df=3298 of=149 ri=1584 ql=6 qs=N.W. kt=0/W b=10 ci=4026154 co=1948 ca=135
rcu_bh:
- 0!c=18446744073709551494 g=18446744073709551494 pq=0 pqc=18446744073709551493 qp=1 dt=86475/1/0 df=11 of=0 ri=0 ql=0 qs=.... b=10 ci=112 co=0 ca=0
- 1!c=18446744073709551496 g=18446744073709551496 pq=1 pqc=18446744073709551495 qp=0 dt=90875/1/0 df=15 of=0 ri=0 ql=0 qs=.... b=10 ci=143 co=0 ca=0
- 2!c=18446744073709551496 g=18446744073709551496 pq=1 pqc=18446744073709551495 qp=0 dt=69661/1/0 df=21 of=0 ri=1 ql=0 qs=.... b=10 ci=88 co=0 ca=0
- 3!c=18446744073709551494 g=18446744073709551494 pq=1 pqc=18446744073709551493 qp=0 dt=70349/1/0 df=13 of=0 ri=0 ql=0 qs=.... b=10 ci=100 co=0 ca=0
- 4!c=18446744073709551494 g=18446744073709551494 pq=0 pqc=18446744073709551493 qp=1 dt=38935/1/0 df=17 of=0 ri=0 ql=0 qs=.... b=10 ci=36 co=0 ca=0
- 5!c=18446744073709551494 g=18446744073709551494 pq=0 pqc=18446744073709551493 qp=1 dt=25127/1/0 df=7 of=0 ri=0 ql=0 qs=.... b=10 ci=32 co=0 ca=0
- 6!c=18446744073709551496 g=18446744073709551496 pq=1 pqc=18446744073709551495 qp=0 dt=22639/1/0 df=9 of=0 ri=0 ql=0 qs=.... b=10 ci=44 co=0 ca=0
- 7 c=182 g=182 pq=1 pqc=181 qp=0 dt=937087/1/0 df=14 of=0 ri=1 ql=0 qs=.... b=10 ci=627 co=0 ca=0
+ 0!c=18446744073709551494 g=18446744073709551494 pq=0 pqc=18446744073709551493 qp=1 dt=86475/1/0 df=11 of=0 ri=0 ql=0 qs=.... kt=0/W b=10 ci=112 co=0 ca=0
+ 1!c=18446744073709551496 g=18446744073709551496 pq=1 pqc=18446744073709551495 qp=0 dt=90875/1/0 df=15 of=0 ri=0 ql=0 qs=.... kt=0/W b=10 ci=143 co=0 ca=0
+ 2!c=18446744073709551496 g=18446744073709551496 pq=1 pqc=18446744073709551495 qp=0 dt=69661/1/0 df=21 of=0 ri=1 ql=0 qs=.... kt=0/W b=10 ci=88 co=0 ca=0
+ 3!c=18446744073709551494 g=18446744073709551494 pq=1 pqc=18446744073709551493 qp=0 dt=70349/1/0 df=13 of=0 ri=0 ql=0 qs=.... kt=0/W b=10 ci=100 co=0 ca=0
+ 4!c=18446744073709551494 g=18446744073709551494 pq=0 pqc=18446744073709551493 qp=1 dt=38935/1/0 df=17 of=0 ri=0 ql=0 qs=.... kt=0/W b=10 ci=36 co=0 ca=0
+ 5!c=18446744073709551494 g=18446744073709551494 pq=0 pqc=18446744073709551493 qp=1 dt=25127/1/0 df=7 of=0 ri=0 ql=0 qs=.... kt=0/W b=10 ci=32 co=0 ca=0
+ 6!c=18446744073709551496 g=18446744073709551496 pq=1 pqc=18446744073709551495 qp=0 dt=22639/1/0 df=9 of=0 ri=0 ql=0 qs=.... kt=0/W b=10 ci=44 co=0 ca=0
+ 7 c=182 g=182 pq=1 pqc=181 qp=0 dt=937087/1/0 df=14 of=0 ri=1 ql=0 qs=.... kt=0/W b=10 ci=627 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
@@ -146,6 +158,23 @@ o "qs" gives an indication of the state of the callback queue
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 slash is zero if there is no work pending and 1 otherwise.
+ The character after the slash 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.
+
+ "Y" The kernel thread is yielding to avoid hogging CPU.
+
+ "?" Unknown value, indicates a bug.
+
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.
@@ -356,6 +385,113 @@ o "nn" is the number of times that this CPU needed nothing. Alert
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
@@ -422,9 +558,9 @@ o "neb" is the number of expedited grace periods that have had
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 12 bits of the jiffies counter in hexadecimal.
+o "j" is the low-order 16 bits of the jiffies counter in hexadecimal.

-o "bt" is the low-order 12 bits of the value that the jiffies counter
+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: