Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756865Ab0HIO4r (ORCPT ); Mon, 9 Aug 2010 10:56:47 -0400 Received: from mail-pw0-f46.google.com ([209.85.160.46]:44447 "EHLO mail-pw0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756687Ab0HIO4q (ORCPT ); Mon, 9 Aug 2010 10:56:46 -0400 Subject: Re: clock drift in set_task_cpu() From: Philby John Reply-To: pjohn@mvista.com To: Peter Zijlstra Cc: Jack Daniel , Ingo Molnar , LKML , Mike Galbraith In-Reply-To: References: <1281002322.1923.1708.camel@laptop> Content-Type: text/plain Date: Mon, 09 Aug 2010 20:26:43 +0530 Message-Id: <1281365803.5390.2.camel@localhost.localdomain> Mime-Version: 1.0 X-Mailer: Evolution 2.24.5 (2.24.5-2.fc10) Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3522 Lines: 93 On Mon, 2010-08-09 at 18:47 +0530, Jack Daniel wrote: > On Thu, Aug 5, 2010 at 3:28 PM, Peter Zijlstra wrote: > > On Wed, 2010-07-21 at 17:10 +0530, Jack Daniel wrote: > >> On a Xeon 55xx with 8 CPU's, I found out the new_rq->clock value is > >> sometimes larger than old_rq->clock and so clock_offset tends to warp > >> around leading to incorrect values. > > > > What values get incorrect, do you observe vruntime funnies or only the > > schedstat values? > > Just the schedstat values, did not observe anything wrong with vruntime. > > > > >> You have very correctly noted in > >> the commit header that all functions that access set_task_cpu() must > >> do so after a call to sched_clock_remote(), in this case the function > >> is sched_fork(). I validated by adding update_rq_clock(old_rq); into > >> set_task_cpu() and that seems to fix the issue. > > > > Ah, so the problem is that task_fork_fair() does the task placement > > without updated rq clocks? In which case I think we should at least do > > an update_rq_clock(rq) in there (see the below patch). > > Yes, this is indeed the problem and your patch seems to fix the issue. > > > > >> But I noticed that > >> since CONFIG_HAVE_UNSTABLE_SCHED_CLOCK is already set, if > >> (sched_clock_stable) in sched_clock_cpu() will yield to true and the > >> flow never gets to sched_clock_remote() or sched_clock_local(). > > > > sched_clock_stable being true implies the clock is stable across cores > > and thus it shouldn't matter. Or are you saying you're seeing it being > > set and still have issues? > > > > Please ignore these comments, initial debugging set me on the wrong > foot, to suggest that TSC is unstable. > > > diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c > > index 9910e1b..f816e74 100644 > > --- a/kernel/sched_fair.c > > +++ b/kernel/sched_fair.c > > @@ -3751,6 +3751,8 @@ static void task_fork_fair(struct task_struct *p) > > > > raw_spin_lock_irqsave(&rq->lock, flags); > > > > + update_rq_clock(rq); > > As you rightly pointed out above, updating the clocks in > task_fork_fair() will rightly fix the issue. Can get rid of rest of > the update_rq_clock() functions as they (like you said), are expensive > and I tested commenting them out. >From 1bc695bc2ac6c941724953b29f6c18196a474b8f Mon Sep 17 00:00:00 2001 From: Philby John Date: Mon, 9 Aug 2010 18:19:08 +0530 Subject: [PATCH] sched: ensure rq->clock get sync'ed when migrating tasks In sched_fork() when we do task placement in ->task_fork_fair() ensure we update_rq_clock() so we work with current time. This has been noted and verified on an Intel Greencity (Xeon 55xx) Signed-off-by: Peter Zijlstra Signed-off-by: Philby John --- kernel/sched_fair.c | 2 +- 1 files changed, 1 insertions(+), 1 deletions(-) diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c index 806d1b2..48bc31c 100644 --- a/kernel/sched_fair.c +++ b/kernel/sched_fair.c @@ -3751,7 +3751,7 @@ static void task_fork_fair(struct task_struct *p) unsigned long flags; raw_spin_lock_irqsave(&rq->lock, flags); - + update_rq_clock(rq); if (unlikely(task_cpu(p) != this_cpu)) __set_task_cpu(p, this_cpu); -- 1.6.3.3.333.g4d53f -- 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/