linux-kernel - Re: bisected: futex regression >= 3.14 - was - Slowdown due to threads bouncing between HT cores

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Message-ID: <1412791184.5173.11.camel@marge.simpson.net>
Date:	Wed, 08 Oct 2014 19:59:44 +0200
From:	Mike Galbraith <umgwanakikbuti@...il.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	"Steinar H. Gunderson" <sgunderson@...foot.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: bisected: futex regression >= 3.14 - was - Slowdown due to
 threads bouncing between HT cores

On Wed, 2014-10-08 at 13:04 -0400, Linus Torvalds wrote: 
> On Wed, Oct 8, 2014 at 11:37 AM, Mike Galbraith
> <umgwanakikbuti@...il.com> wrote:
> >
> > 11d4616bd07f38d496bd489ed8fad1dc4d928823 is the first bad commit
> > commit 11d4616bd07f38d496bd489ed8fad1dc4d928823
> > Author: Linus Torvalds <torvalds@...ux-foundation.org>
> > Date:   Thu Mar 20 22:11:17 2014 -0700
> >
> >     futex: revert back to the explicit waiter counting code
> 
> While that revert might make things a tiny bit slower (I hated doing
> it, but the clever approach sadly didn't work on powerpc and depended
> on x86 locking semantics), I seriously doubt it's really relevant.
> It's more likely that the *real* problem itself is very
> timing-dependent, and the subtle synchronization changes here then
> expose it or hide it, rather than really fixing anything.
> 
> So like Thomas, I would suspect a race condition in the futex use, and
> then the exact futex implementation details are just exposing it
> incidentally.

Whew, good, futex.c is hard.  Heads up chess guys <punt>. 

-Mike

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