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Date: Fri, 24 Oct 2014 18:38:41 +0200 From: Mike Galbraith <umgwanakikbuti@...il.com> To: Thomas Gleixner <tglx@...utronix.de> Cc: Linus Torvalds <torvalds@...ux-foundation.org>, "Steinar H. Gunderson" <sgunderson@...foot.com>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, Peter Zijlstra <peterz@...radead.org> Subject: Re: bisected: futex regression >= 3.14 - was - Slowdown due to threads bouncing between HT cores On Fri, 2014-10-24 at 17:25 +0200, Thomas Gleixner wrote: > 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@...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>. > > I wonder whether the barrier fix which got into 3.17 late fixes that > issue as well. Yes, it did. -Mike -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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