linux-kernel - Re: [PATCH 06/40] sched: add wakeup/sleep sched

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Date:	Tue, 19 Jan 2010 09:55:02 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Tejun Heo <tj@...nel.org>
Cc:	torvalds@...ux-foundation.org, mingo@...e.hu, awalls@...ix.net,
	linux-kernel@...r.kernel.org, jeff@...zik.org,
	akpm@...ux-foundation.org, jens.axboe@...cle.com,
	rusty@...tcorp.com.au, cl@...ux-foundation.org,
	dhowells@...hat.com, arjan@...ux.intel.com, avi@...hat.com,
	johannes@...solutions.net, andi@...stfloor.org,
	Mike Galbraith <efault@....de>
Subject: Re: [PATCH 06/40] sched: add wakeup/sleep sched_notifiers and
 allow NULL notifier ops

On Tue, 2010-01-19 at 17:28 +0900, Tejun Heo wrote:
> Hello,
> 
> On 01/19/2010 10:04 AM, Tejun Heo wrote:
> >> I'm thinking that we can place it next to activate_task(), if it makes
> >> you feel better you can place them both at the end up ttwu_activate()
> >> instead of in the middle.
> >>
> >> Esp. with the callback you have it really doesn't matter.
> > 
> > Alright, if it's safe, there's no reason to keep it separate with an
> > extra branch.  I'll move it.
> 
> Alright, was trying to convert it and I'm still a bit worried.  One of
> the reasons I put it at the end of post_activation() is to allow
> calling try_to_wake_up_local() from wakeup callback.  This won't be
> used by cmwq right now but making it symmetrical to sleep callback
> would be more consistent, so...  If we fire wakeup callback right
> after activate_task() and allow try_to_wake_up_local() to be called
> from it, wake up logic ends up being nested inside outer wake up which
> is still in progress.  Would that be safe too?

I think so, still doing a wakeup from a wakeup sounds like trouble in
that it has the potential to a thundering herd, so I'd really rather
you'd not do something like that.

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