Messages in this thread |  | | | Date | Fri, 25 Jul 2008 15:20:08 +0200 | | From | "Dmitry Adamushko" <> | | Subject | Re: [patch, rfc: 2/2] sched, hotplug: ensure a task is on the valid cpu after set_cpus_allowed_ptr() |
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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.
--- 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().
> >> Signed-off-by: Dmitry Adamushko <dmitry.adamushko@gmail.com> >> >> diff --git a/kernel/sched.c b/kernel/sched.c >> index b4ccc8b..c3bd78a 100644 >> --- a/kernel/sched.c >> +++ b/kernel/sched.c >> @@ -5774,21 +5774,23 @@ int set_cpus_allowed_ptr(struct task_struct *p, const cpumask_t *new_mask) >> } >> >> /* Can the task run on the task's current CPU? If so, we're done */ >> - if (cpu_isset(task_cpu(p), *new_mask)) >> - goto out; >> + while (!cpu_isset(task_cpu(p), p->cpus_allowed)) { >> + int cpu = any_online_cpu(p->cpus_allowed); >> >> - if (migrate_task(p, any_online_cpu(*new_mask), &req)) { >> - /* Need to wait for migration thread (might exit: take ref). */ >> - struct task_struct *mt = rq->migration_thread; >> + if (migrate_task(p, cpu, &req)) { >> + /* Need to wait for migration thread (might exit: take ref). */ >> + struct task_struct *mt = rq->migration_thread; >> >> - get_task_struct(mt); >> - task_rq_unlock(rq, &flags); >> - wake_up_process(mt); >> - put_task_struct(mt); >> + get_task_struct(mt); >> + task_rq_unlock(rq, &flags); >> + wake_up_process(mt); >> + put_task_struct(mt); >> >> - wait_for_completion(&req.done); >> - tlb_migrate_finish(p->mm); >> - return 0; >> + wait_for_completion(&req.done); >> + tlb_migrate_finish(p->mm); >> + >> + rq = task_rq_lock(p, &flags); >> + } >> } >> out: >> task_rq_unlock(rq, &flags); >> >> > >
-- Best regards, Dmitry Adamushko
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