Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757298AbYJHXNX (ORCPT ); Wed, 8 Oct 2008 19:13:23 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754355AbYJHXNN (ORCPT ); Wed, 8 Oct 2008 19:13:13 -0400 Received: from casper.infradead.org ([85.118.1.10]:51063 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750904AbYJHXNM (ORCPT ); Wed, 8 Oct 2008 19:13:12 -0400 Subject: Re: [PATCH] sched_clock: prevent scd->clock from moving backwards From: Peter Zijlstra To: Dave Kleikamp Cc: Jeremy Fitzhardinge , Steven Rostedt , Ingo Molnar , Linux Kernel Mailing List In-Reply-To: <1223470854.6336.15.camel@norville.austin.ibm.com> References: <48D959E8.4000303@goop.org> <1223470773.6336.13.camel@norville.austin.ibm.com> <1223470854.6336.15.camel@norville.austin.ibm.com> Content-Type: text/plain Date: Thu, 09 Oct 2008 01:05:04 +0200 Message-Id: <1223507104.7382.6.camel@lappy.programming.kicks-ass.net> Mime-Version: 1.0 X-Mailer: Evolution 2.22.3.1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2245 Lines: 58 On Wed, 2008-10-08 at 08:00 -0500, Dave Kleikamp wrote: > sched_clock: prevent scd->clock from moving backwards > > When sched_clock_cpu() couples the clocks between two cpus, it may > increment scd->clock beyond the GTOD tick window that __update_sched_clock() > uses to clamp the clock. A later call to __update_sched_clock() may move > the clock back to scd->tick_gtod + TICK_NSEC, violating the clock's > monotonic property. > > This patch ensures that scd->clock will not be set backward. Ah, yes indeed, this comes from the tick not happening at the same time on different cpus, so if we use a local timestamp to move a remote clock forward, this scenario could indeed happen. The fix looks good to me, good catch, thanks shaggy! A related 'fix' which I'm still not quite sure about is making the window 'tick_gtod + 2*TICK_NSEC'. That increases the max observed difference to 4 jiffies, but allows ticks to be 'late' a bit without first holding back time and then jumping ahead again. > Signed-off-by: Dave Kleikamp > Cc: Ingo Molnar > Cc: Peter Zijlstra Acked-by: Peter Zijlstra > > diff --git a/kernel/sched_clock.c b/kernel/sched_clock.c > index e8ab096..a989d64 100644 > --- a/kernel/sched_clock.c > +++ b/kernel/sched_clock.c > @@ -118,13 +118,13 @@ static u64 __update_sched_clock(struct sched_clock_data *scd, u64 now) > > /* > * scd->clock = clamp(scd->tick_gtod + delta, > - * max(scd->tick_gtod, scd->clock), > - * scd->tick_gtod + TICK_NSEC); > + * max(scd->tick_gtod, scd->clock), > + * min(scd->clock, scd->tick_gtod + TICK_NSEC)); > */ > > clock = scd->tick_gtod + delta; > min_clock = wrap_max(scd->tick_gtod, scd->clock); > - max_clock = scd->tick_gtod + TICK_NSEC; > + max_clock = wrap_min(scd->clock, scd->tick_gtod + TICK_NSEC); > > clock = wrap_max(clock, min_clock); > clock = wrap_min(clock, max_clock); > > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/