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Subject: Re: [RFC][PATCH 16/17] sched: Move the second half of ttwu() to
 the remote cpu
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Chris Mason <chris.mason@oracle.com>,
        Frank Rowand <frank.rowand@am.sony.com>, Ingo Molnar <mingo@elte.hu>,
        Thomas Gleixner <tglx@linutronix.de>, Mike Galbraith <efault@gmx.de>,
        Paul Turner <pjt@google.com>, Jens Axboe <axboe@kernel.dk>,
        Yong Zhang <yong.zhang0@gmail.com>, linux-kernel@vger.kernel.org
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Date: Tue, 04 Jan 2011 16:43:01 +0100
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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
anything.

> 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.