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" : > > >  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 > > >  Cc: Ingo Molnar > > >  Signed-off-by: Peter Zijlstra > > >  --- > > >   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..