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From: Linus Torvalds <torvalds@linux-foundation.org>
To: Mike Galbraith <umgwanakikbuti@gmail.com>
Cc: "Steinar H. Gunderson" <sgunderson@bigfoot.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Thomas Gleixner <tglx@linutronix.de>
Subject: Re: bisected: futex regression >= 3.14 - was - Slowdown due to threads bouncing between HT cores
Date: Wed, 8 Oct 2014 13:04:01 -0400	[thread overview]
Message-ID: <CA+55aFyuH07zzD6dD4k0pdyvJ7fUhFdFrmPYJi9W=FtHihgndA@mail.gmail.com> (raw)
In-Reply-To: <1412782664.5179.75.camel@marge.simpson.net>

On Wed, Oct 8, 2014 at 11:37 AM, Mike Galbraith
<umgwanakikbuti@gmail.com> wrote:
>
> 11d4616bd07f38d496bd489ed8fad1dc4d928823 is the first bad commit
> commit 11d4616bd07f38d496bd489ed8fad1dc4d928823
> Author: Linus Torvalds <torvalds@linux-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

  parent reply	other threads:[~2014-10-08 17:04 UTC|newest]

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2014-10-03 19:44 Slowdown due to threads bouncing between HT cores Steinar H. Gunderson
2014-10-03 21:11 ` Marc Burkhardt
2014-10-03 21:14   ` Steinar H. Gunderson
2014-10-04  9:22     ` Marc Burkhardt
2014-10-04 13:41 ` Andi Kleen
2014-10-04 14:12   ` Steinar H. Gunderson
2014-10-04 14:50 ` Chuck Ebbert
2014-10-05 11:19   ` Steinar H. Gunderson
2014-10-08 15:37 ` bisected: futex regression >= 3.14 - was - " Mike Galbraith
2014-10-08 16:14   ` Thomas Gleixner
2014-10-08 16:45     ` Steinar H. Gunderson
2014-10-08 17:52       ` Mike Galbraith
2014-10-08 16:23   ` Steinar H. Gunderson
2014-10-08 17:04   ` Linus Torvalds [this message]
2014-10-08 17:05     ` Steinar H. Gunderson
2014-10-08 17:59     ` Mike Galbraith
2014-10-24 15:25       ` Thomas Gleixner
2014-10-24 16:38         ` Mike Galbraith
2014-10-26 10:39           ` Steinar H. Gunderson
2014-10-26 13:16             ` Mike Galbraith
2014-10-26 13:58               ` Mike Galbraith
2014-10-26 14:11                 ` Steinar H. Gunderson
2014-10-26 14:41                   ` Mike Galbraith
2014-10-27 10:05                   ` Mike Galbraith

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