From: Thomas Gleixner <tglx@linutronix.de>
To: Mike Galbraith <umgwanakikbuti@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
"Steinar H. Gunderson" <sgunderson@bigfoot.com>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Peter Zijlstra <peterz@infradead.org>
Subject: Re: bisected: futex regression >= 3.14 - was - Slowdown due to threads bouncing between HT cores
Date: Fri, 24 Oct 2014 17:25:16 +0200 (CEST) [thread overview]
Message-ID: <alpine.DEB.2.11.1410241724370.5308@nanos> (raw)
In-Reply-To: <1412791184.5173.11.camel@marge.simpson.net>
On Wed, 8 Oct 2014, Mike Galbraith wrote:
> On Wed, 2014-10-08 at 13:04 -0400, Linus Torvalds wrote:
> > 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.
>
> Whew, good, futex.c is hard. Heads up chess guys <punt>.
I wonder whether the barrier fix which got into 3.17 late fixes that
issue as well.
Thanks,
tglx
next prev parent reply other threads:[~2014-10-24 15:25 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
2014-10-08 17:05 ` Steinar H. Gunderson
2014-10-08 17:59 ` Mike Galbraith
2014-10-24 15:25 ` Thomas Gleixner [this message]
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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