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Date:   Thu, 9 May 2019 12:36:25 -0700
From:   "Paul E. McKenney" <paulmck@linux.ibm.com>
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        linux-kernel@vger.kernel.org, andrea.parri@amarulasolutions.com
Subject: Re: Question about sched_setaffinity()
Reply-To: paulmck@linux.ibm.com
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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@linux.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