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

[Peter Zijlstra <peterz@infradead.org>]

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.