linux-kernel - Re: Question about sched

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Message-Id: <20190509221730.GM3923@linux.ibm.com>
Date:   Thu, 9 May 2019 15:17:30 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Andrea Parri <andrea.parri@...rulasolutions.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Joel Fernandes <joelaf@...gle.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: Re: Question about sched_setaffinity()

On Thu, May 09, 2019 at 11:56:35PM +0200, Andrea Parri wrote:
> On Thu, May 09, 2019 at 11:40:25PM +0200, Andrea Parri wrote:
> > 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?
> > 
> > And I can finally see/reproduce it!!
> > 
> > Frankly, no idea how this is happening (I have been staring at these
> > traces/functions for hours/days now...  ;-/ )
> > 
> > Adding some "sched" folks in Cc: hopefully, they can shed some light
> > about this.
> 
> +Thomas, +Sebastian
> 
> Thread starts here:
> 
> http://lkml.kernel.org/r/20190427180246.GA15502@linux.ibm.com

Peter Zijlstra kindly volunteered over IRC to look at this more closely
tomorrow (well, today, his time).  It is quite strange, though!  ;-)

							Thanx, Paul

>   Andrea
> 
> 
> > 
> > And yes, my only suggestion/approach would be to keep bisecting this
> > code with printk*..., 'fun' ;-/
> > 
> >   Andrea
>