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Message-Id: <20190509193625.GA12455@linux.ibm.com>
Date: Thu, 9 May 2019 12:36:25 -0700
From: "Paul E. McKenney" <paulmck@...ux.ibm.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Peter Zijlstra <peterz@...radead.org>,
linux-kernel@...r.kernel.org, andrea.parri@...rulasolutions.com
Subject: Re: Question about sched_setaffinity()
On Thu, May 09, 2019 at 10:36:54AM -0700, Paul E. McKenney wrote:
> On Tue, May 07, 2019 at 03:16:13PM -0700, Paul E. McKenney wrote:
> > On Wed, May 01, 2019 at 01:27:13PM -0700, Paul E. McKenney wrote:
> > > On Wed, May 01, 2019 at 03:16:55PM -0400, Steven Rostedt wrote:
> > > > On Wed, 1 May 2019 12:12:13 -0700
> > > > "Paul E. McKenney" <paulmck@...ux.ibm.com> wrote:
> > > >
> > > >
> > > > > OK, what I did was to apply the patch at the end of this email to -rcu
> > > > > branch dev, then run rcutorture as follows:
> > > > >
> > > > > nohup tools/testing/selftests/rcutorture/bin/kvm.sh --cpus 8 --duration 2 --configs "TRIVIAL" --bootargs "trace_event=sched:sched_switch,sched:sched_wakeup ftrace=function_graph ftrace_graph_filter=sched_setaffinity,migration_cpu_stop"
> > > > >
> > > > > This resulted in the console output that I placed here:
> > > > >
> > > > > http://www2.rdrop.com/~paulmck/submission/console.log.gz
> > > > >
> > > > > But I don't see calls to sched_setaffinity() or migration_cpu_stop().
> > > > > Steve, is something else needed on the kernel command line in addition to
> > > > > the following?
> > > > >
> > > > > ftrace=function_graph ftrace_graph_filter=sched_setaffinity,migration_cpu_stop
> > > >
> > > > Do you have function graph enabled in the config?
> > > >
> > > > [ 2.098303] ftrace bootup tracer 'function_graph' not registered.
> > >
> > > I guess I don't! Thank you, will fix.
> > >
> > > Let's see...
> > >
> > > My .config has CONFIG_HAVE_FUNCTION_GRAPH_TRACER=y. It looks like I
> > > need CONFIG_FUNCTION_GRAPH_TRACER=y, which I don't have. And it looks
> > > like that needs CONFIG_FUNCTION_TRACER=y, which I also don't have.
> > > But I do have CONFIG_HAVE_FUNCTION_TRACER=y. So I should add this
> > > to my rcutorture command line:
> > >
> > > --kconfig "CONFIG_FUNCTION_TRACER=y CONFIG_FUNCTION_GRAPH_TRACER=y".
> > >
> > > I fired this up. Here is hoping! ;-)
> > >
> > > And it does have sched_setaffinity(), woo-hoo!!! I overwrote the old file:
> > >
> > > http://www2.rdrop.com/~paulmck/submission/console.log.gz
> >
> > And I reran after adding a trace_printk(), which shows up as follows:
> >
> > [ 211.409565] 6) | /* On unexpected CPU 6, expected 4!!! */
> >
> > I placed the console log here:
> >
> > http://www2.rdrop.com/~paulmck/submission/console.tpk.log.gz
> >
> > Just in case the earlier log proves useful.
> >
> > And it is acting as if the destination CPU proved to be offline. Except
> > that this rcutorture scenario doesn't offline anything, and I don't see
> > any CPU-hotplug removal messages. So I added another trace_printk() to
> > print out cpu_online_mask. This gets me the following:
> >
> > [ 31.565605] 0) | /* On unexpected CPU 0, expected 1!!! */
> > [ 31.565605] 0) | /* Online CPUs: 0-7 */
> >
> > So we didn't make it to CPU 1 despite its being online. I placed the
> > console log here:
> >
> > http://www2.rdrop.com/~paulmck/submission/console.tpkol.log.gz
> >
> > Thoughts?
> >
> > Updated patch against -rcu below in case it is useful.
>
> I added more debug and got this:
>
> [ 215.097318] 4) | /* On unexpected CPU 4, expected 5!!! */
> [ 215.098285] 4) | /* Online CPUs: 0-7 Active CPUs: 0-7 */
> [ 215.099283] 4) | /* ret = 0, ->cpus_allowed 0-4,6-7 */
>
> http://www.rdrop.com/~paulmck/submission/console.tpkm.log.gz
>
> The task's ->cpus_allowed got set to the bitwise not of the requested
> mask. I took a quick scan through the code but don't see how this
> happens.
>
> Very strange.
>
> Any suggestions for further debugging? Left to myself, I would copy
> the requested cpumask to a per-task location and insert checks in the
> sched_setaffinity() code path, crude and ugly though that might seem.
> (Ugly enough that I will wait until tomorrow to try it out.)
And of course the nicer approach is to simply drop a few trace_printk()
calls into sched_setaffinity(), which got me this (with other CPU
tracing removed):
[ 207.440153] 0) | /* sched_setaffinity: new_mask 5 */
[ 207.445567] 0) | /* On unexpected CPU 0, expected 5!!! */
[ 207.450017] 0) | /* Online CPUs: 0-7 Active CPUs: 0-7 */
[ 207.454943] 0) | /* ret = 0, ->cpus_allowed 0-4,6-7 */
http://www.rdrop.com/~paulmck/submission/console.tkpms.log.gz
(Search for "->cpus_allowed".)
I forward-ported the relevant patches from -rcu and placed them on -rcu
branch peterz.2019.05.09a, and this is what produced the output above.
Any other debugging thoughts?
Or, if you wish, you can reproduce by running the following:
nohup tools/testing/selftests/rcutorture/bin/kvm.sh --cpus 8 --duration 2 --configs "TRIVIAL" --bootargs "trace_event=sched:sched_switch,sched:sched_wakeup ftrace=function_graph ftrace_graph_filter=sched_setaffinity,migration_cpu_stop" --kconfig "CONFIG_FUNCTION_TRACER=y CONFIG_FUNCTION_GRAPH_TRACER=y"
This gets me the following summary output:
--- Thu May 9 12:08:31 PDT 2019 Test summary:
Results directory: /home/git/linux-2.6-tip/tools/testing/selftests/rcutorture/res/2019.05.09-12:08:31
tools/testing/selftests/rcutorture/bin/kvm.sh --cpus 8 --duration 2 --configs TRIVIAL --bootargs trace_event=sched:sched_switch,sched:sched_wakeup ftrace=function_graph ftrace_graph_filter=sched_setaffinity,migration_cpu_stop --kconfig CONFIG_FUNCTION_TRACER=y CONFIG_FUNCTION_GRAPH_TRACER=y
TRIVIAL ------- 2177 GPs (18.1417/s) [trivial: g0 f0x0 ]
:CONFIG_HOTPLUG_CPU: improperly set
WARNING: BAD SEQ 2176:2176 last:2177 version 4
/home/git/linux-2.6-tip/tools/testing/selftests/rcutorture/res/2019.05.09-12:08:31/TRIVIAL/console.log
WARNING: Assertion failure in /home/git/linux-2.6-tip/tools/testing/selftests/rcutorture/res/2019.05.09-12:08:31/TRIVIAL/console.log
WARNING: Summary: Warnings: 1 Bugs: 1 Call Traces: 5 Stalls: 8
Thanx, Paul
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