linux-kernel - Re: [RFC][PATCH] sched: Avoid select_idle_sibling() for wake

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Message-ID: <20130926154303.GA3364@laptop.programming.kicks-ass.net>
Date:	Thu, 26 Sep 2013 17:43:03 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Paul Turner <pjt@...gle.com>
Cc:	Mike Galbraith <bitbucket@...ine.de>,
	Ingo Molnar <mingo@...nel.org>, Rik van Riel <riel@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH] sched: Avoid select_idle_sibling() for
 wake_affine(.sync=true)

On Thu, Sep 26, 2013 at 04:35:33PM +0200, Peter Zijlstra wrote:
> On Thu, Sep 26, 2013 at 04:39:30AM -0700, Paul Turner wrote:
> > It is my intuition that there are a few common objects with fairly
> > polarized behavior:  I.e. For condition variables and producer
> > consumer queues, a wakeup strongly predicts blocking.  Whereas for
> > locks protecting objects, e.g. a Mutex, would be expected to have the
> > opposite behavior.
> 
> Agreed; however none of those seem to have the property we're looking
> for.
> 
> Even produces consumer queues on their own don't generate the
> alternating patterns we're looking for with the SYNC hint.
> 
> We need a 'guarantee' that the waker is going to stop until the wakee is
> done.
> 
> What we're looking for is the typical synchronous request-reply like
> pattern -- and that doesn't seem to correlate to any one locking object.
> 
> Rather it is an inter-task relation; so task state does make sense in
> finding them. We could for instance try and infer which task is
> servicing requests; and then we know that requesting tasks will sleep
> until reply.
> 

Oh never mind, I see what you meant, the edges in that graph are the
locks.

Can't use RIPs for futexes though; you'd likely end up in the one
pthread_mutex_lock() implementation or such.
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