Messages in this thread | | | Subject | Re: [patch, rfc: 2/2] sched, hotplug: ensure a task is on the valid cpu after set_cpus_allowed_ptr() | From | Peter Zijlstra <> | Date | Fri, 25 Jul 2008 15:39:44 +0200 |
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On Fri, 2008-07-25 at 15:20 +0200, Dmitry Adamushko wrote: > 2008/7/25 Peter Zijlstra <a.p.zijlstra@chello.nl>: > > On Fri, 2008-07-25 at 00:15 +0200, Dmitry Adamushko wrote: > >> > >> From: Dmitry Adamushko <dmitry.adamushko@gmail.com> > >> Subject: sched, hotplug: ensure a task is on the valid cpu after > >> set_cpus_allowed_ptr() > >> > >> --- > >> sched, hotplug: ensure a task is on the valid cpu after set_cpus_allowed_ptr() > >> > >> The 'new_mask' may not include task_cpu(p) so we migrate 'p' on another 'cpu'. > >> In case it can't be placed on this 'cpu' immediately, we submit a request > >> to the migration thread and wait for its completion. > >> > >> Now, by the moment this request gets handled by the migration_thread, > >> 'cpu' may well be offline/non-active. As a result, 'p' continues > >> running on its old cpu which is not in the 'new_mask'. > >> > >> Fix it: ensure 'p' ends up on a valid cpu. > >> > >> Theoreticaly (but unlikely), we may get an endless loop if someone cpu_down()'s > >> a new cpu we have choosen on each iteration. > >> > >> Alternatively, we may introduce a special type of request to migration_thread, > >> namely "move_to_any_allowed_cpu" (e.g. by specifying dest_cpu == -1). > >> > >> Note, any_active_cpu() instead of any_online_cpu() would be better here. > > > > Hrmm,.. this is all growing into something of a mess.. defeating the > > whole purpose of introducing that cpu_active_map stuff. > > > > Would the suggested SRCU logic simplify all this? > > Ah, wait a second. > > sched_setaffinity() -> set_cpus_allowed_ptr() is ok vs. cpu_down() as > it does use get_online_cpus(). So none of the cpus can become offline > while we are in set_cpus_allowed_ptr(). > > but there are numerous calls to set_cpus_allowed_ptr() from other > places and not all of them seem to call get_online_cpus()... > > yeah, I should check this issue again.. > > btw., indeed all these different sync. cases are a bit of mess.
Will ponder it a bit more, but my brain can't seem to let go of SRCU now.. I'll go concentrate on making the swap-over-nfs patches prettier, maybe that will induce a brainwave ;-)
> --- > > btw., I was wondering about this change: > > ba42059fbd0aa1ac91b582412b5fedb1258f241f > > sched: hrtick_enabled() should use cpu_active() > > Peter pointed out that hrtick_enabled() should use cpu_active().
What exactly were you wondering about?
It seemed a good idea to stop starting hrtimers before we migrate them to another cpu (one of the things done later in cpu_down), thereby avoiding spurious fires on remote cpus.
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