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Date: Wed, 27 Jun 2001 14:57:43 -0700
From: Scott Long <scott@swiftview.com>
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To: linux-kernel@vger.kernel.org
Subject: Re: wake_up vs. wake_up_sync
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Does reschedule_idle() ever cause the current CPU to get scheduled? That
is, if someone calls wake_up() and wakes up a higher-priority process
could reschedule_idle() potentially immediately switch the current CPU
to that higher-priority process?

Because this is NOT what I want to happen (it would produce a deadlock
in this particular situation). Having other CPUs get scheduled is ok,
but having the process that called wake_up() get kicked out would be
very bad. In that case I suppose I will have to use wake_up_sync().

Would the following be an appropriate solution?

{
    wake_up_sync(&wq->q);

    /* Potential deadlock situation */
    user_unlock(&wq->lock);

    /* Potential for deadlock has passed */
    reschedule_idle();
}

Thanks,
Scott

Manfred Spraul wrote:
> 
> > I'm having trouble understanding the difference between these.
> > Synchronous apparently causes try_to_wake_up() to NOT call
> > reschedule_idle() but I'm uncertain what reschedule_idle() is doing. I
> > assume it just looks for an idle CPU and makes that CPU reschedule.
> >
> > What is the purpose of wake_up_sync?
> 
> Avoid the reschedule_idle() call - it's quite costly, and it could cause
> processes jumping from one cpu to another.
> 
> > Why would you want to prevent
> > reschedule_idle()?
> >
> If one process runs, wakes up another process and _knows_ that it's
> going to sleep immediately after the wake_up it doesn't need the
> reschedule_idle: the current cpu will be idle soon, the scheduler
> doesn't need to find another cpu for the woken up thread.
> 
> I think the pipe code is the only user of _sync right now - pipes cause
> an incredible amount of task switches.
> 
> --
>         Manfred