Re: [PATCH] sched: Fix migration_cpu_stop() return value

Peter Zijlstra
2014-06-04T12:29:18Z

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On Wed, Jun 04, 2014 at 02:03:38PM +0200, Peter Zijlstra wrote:
> On Wed, Jun 04, 2014 at 03:18:45PM +0400, Kirill Tkhai wrote:
> > Hi, Peter,
> >
> > 04.06.2014, 14:41, "Peter Zijlstra" <peterz@infradead.org>:
> > >  A while ago I did a similar patch for some debugging, but looking at it
> > >  again today I realized we should probably fix this anyway.
> > >
> > >  ---
> > >  Subject: sched: Fix migration_cpu_stop() return value
> > >
> > >  There are a number of migration_cpu_stop() users; and some actually care
> > >  about the success of the migration. So report this.
> > >
> > >  In particular migrate_task_to() as used from task_numa_migrate()
> > >  actually tests this return value.
> > >
> > >  Also change set_cpus_allowed_ptr() to propagate this return value, since
> > >  it already returns other errors.
> > >
> > >  Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
> > >  Cc: Ingo Molnar <mingo@redhat.com>
> > >  Signed-off-by: Peter Zijlstra <peterz@infradead.org>
> > >  ---
> > >   kernel/sched/core.c |   15 +++++++++++----
> > >   1 file changed, 11 insertions(+), 4 deletions(-)
> > (snipped everything because of bad email editor)
> >
> > In set_cpus_allowed_ptr() p->on_rq branch can not fail.
> >
> > We've changed affinity and released rq's lock, so task can migrate
> > on allowed cpu only (even if migration_cpu_stop fails).
> >
> > And it's a little ambiguously how user should react on this EAGAIN.
>
> Try again? So one reason it might fail is because the task got migrated
> in between the stop_cpu_call(migration_cpu_stop) call getting to
> __migrate_task().
>
> Esp. if you look at migrate_task_to() its fairly easy to fail this.
> Currently it reports success, even though we completely failed to
> migrate.
>
> On -EAGAIN, re-evaluate the target and try again (later). Like for the
> numa case, we'll try again on the next task_numa_migrate() call if its
> still relevant.

Argh, I just realized that the one case the numa code is interested in
is actually reported as success (task_cpu() != cpu).

I'll go make a new patch..

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