Re: [RFC][PATCH 16/17] sched: Move the second half of ttwu() tothe remote cpu

From: Peter Zijlstra
Date: Tue Jan 04 2011 - 10:43:12 EST

On Tue, 2011-01-04 at 16:18 +0100, Oleg Nesterov wrote:

> > I don't think so, nobody should be migrating a TASK_WAKING task.
> I am not sure...
> Suppose that p was TASK_INTERRUPTIBLE and p->on_rq == 1 before, when
> set_cpus_allowed_ptr() was called. To simplify, suppose that
> the caller is preempted right after it drops p->pi_lock and before
> it does stop_one_cpu(migration_cpu_stop).
> After that p can complete chedule() and deactivate itself.
> Now, try_to_wake_up() can set TASK_WAKING, choose another CPU,
> and do ttwu_queue_remote().
> Finally, the caller of set_cpus_allowed_ptr() resumes and
> schedules migration_cpu_stop.

But __migrate_task() will then find !p->on_rq and not actually do

> It is very possible I missed something, but what is the new
> locking rules for set_task_cpu() anyway? I mean, which rq->lock
> it needs?

In the patches I just posted: set_task_cpu(p, cpu) callers need to
either hold task_rq(p)->lock (the old rq) or p->pi_lock.

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