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

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Message-ID: <CA+55aFyuH07zzD6dD4k0pdyvJ7fUhFdFrmPYJi9W=FtHihgndA@mail.gmail.com>
Date:	Wed, 8 Oct 2014 13:04:01 -0400
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Mike Galbraith <umgwanakikbuti@...il.com>
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, 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.

               Linus
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