From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1765820AbZARIia (ORCPT ); Sun, 18 Jan 2009 03:38:30 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1755138AbZARIiV (ORCPT ); Sun, 18 Jan 2009 03:38:21 -0500 Received: from mx2.mail.elte.hu ([157.181.151.9]:42775 "EHLO mx2.mail.elte.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752148AbZARIiU (ORCPT ); Sun, 18 Jan 2009 03:38:20 -0500 Date: Sun, 18 Jan 2009 09:37:44 +0100 From: Ingo Molnar To: Avi Kivity Cc: Mike Galbraith , Kevin Shanahan , Andrew Morton , a.p.zijlstra@chello.nl, torvalds@linux-foundation.org, linux-kernel@vger.kernel.org Subject: Re: [git pull] scheduler fixes Message-ID: <20090118083744.GB21940@elte.hu> References: <1232173776.7073.21.camel@marge.simson.net> <1232186054.6813.48.camel@marge.simson.net> <1232186877.14073.59.camel@laptop> <1232188484.6813.85.camel@marge.simson.net> <1232193617.14073.67.camel@laptop> <1232194752.6273.5.camel@marge.simson.net> <20090117044316.bda7d0bd.akpm@linux-foundation.org> <1232198574.16303.8.camel@marge.simson.net> <20090117160115.GA31601@elte.hu> <4972E860.5080206@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4972E860.5080206@redhat.com> User-Agent: Mutt/1.5.18 (2008-05-17) X-ELTE-VirusStatus: clean X-ELTE-SpamScore: -1.5 X-ELTE-SpamLevel: X-ELTE-SpamCheck: no X-ELTE-SpamVersion: ELTE 2.0 X-ELTE-SpamCheck-Details: score=-1.5 required=5.9 tests=BAYES_00 autolearn=no SpamAssassin version=3.2.3 -1.5 BAYES_00 BODY: Bayesian spam probability is 0 to 1% [score: 0.0000] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Avi Kivity wrote: > Ingo Molnar wrote: >> * Mike Galbraith wrote: >> >> >>> On Sat, 2009-01-17 at 04:43 -0800, Andrew Morton wrote: >>> >>>> http://bugzilla.kernel.org/show_bug.cgi?id=12465 just popped up - another >>>> scheduler regression. It has been bisected. >>>> >>> Seems pretty clear. I'd suggest reverting it. >>> >> >> We can revert it (and will revert it if no solution is found), but i'd >> also like to understand why it happens, because that kind of regression >> from this change is unexpected - we might be hiding some bug that could >> pop up under less debuggable circumstances, so we need to understand it >> while we have a chance. >> >> Below is the commit in question. Avi, any ideas what makes KVM special >> here? Perhaps its use of "preempt notifiers" is causing a problem >> somehow? >> > > preempt notifiers use should cause additional context switch costs of a > few thousand cycles and possible an IPI (if a vcpu was migrated). So > I'd suspect scheduling latency here. > > Is it possible to trace this (the time between a wake up and actual > scheduling of a task)? Can you reproduce those latencies? We didnt get similar reports from elsewhere so there seems to be a KVM angle. Ingo