From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753608AbbIRLbF (ORCPT ); Fri, 18 Sep 2015 07:31:05 -0400 Received: from e06smtp13.uk.ibm.com ([195.75.94.109]:34405 "EHLO e06smtp13.uk.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753057AbbIRLbC (ORCPT ); Fri, 18 Sep 2015 07:31:02 -0400 X-Helo: d06dlp03.portsmouth.uk.ibm.com X-MailFrom: dingel@linux.vnet.ibm.com X-RcptTo: stable@vger.kernel.org Date: Fri, 18 Sep 2015 13:30:54 +0200 From: Dominik Dingel To: Paolo Bonzini Cc: Peter Zijlstra , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Ingo Molnar , Tim Chen , stable@vger.kernel.org Subject: Re: [PATCH] sched: access local runqueue directly in single_task_running Message-ID: <20150918133054.0f84b43e@BR9TG4T3.de.ibm.com> In-Reply-To: <55FBF4FD.6060609@redhat.com> References: <1442568465-71559-1-git-send-email-dingel@linux.vnet.ibm.com> <55FBF4FD.6060609@redhat.com> Organization: IBM X-Mailer: Claws Mail 3.9.3 (GTK+ 2.24.23; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 15091811-0013-0000-0000-0000056CEE98 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 18 Sep 2015 13:26:53 +0200 Paolo Bonzini wrote: > > > On 18/09/2015 11:27, Dominik Dingel wrote: > > Commit 2ee507c47293 ("sched: Add function single_task_running to let a task > > check if it is the only task running on a cpu") referenced the current > > runqueue with the smp_processor_id. When CONFIG_DEBUG_PREEMPT is enabled, > > that is only allowed if preemption is disabled or the currrent task is > > bound to the local cpu (e.g. kernel worker). > > > > With commit f78195129963 ("kvm: add halt_poll_ns module parameter") KVM > > calls single_task_running. If CONFIG_DEBUG_PREEMPT is enabled that > > generates a lot of kernel messages. > > > > To avoid adding preemption in that cases, as it would limit the usefulness, > > we change single_task_running to access directly the cpu local runqueue. > > > > Cc: Tim Chen > > Suggested-by: Peter Zijlstra > > Cc: # 4.2.x > > Signed-off-by: Dominik Dingel > > --- > > kernel/sched/core.c | 8 ++++---- > > 1 file changed, 4 insertions(+), 4 deletions(-) > > > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > > index 78b4bad10..5bfad0b 100644 > > --- a/kernel/sched/core.c > > +++ b/kernel/sched/core.c > > @@ -2614,13 +2614,13 @@ unsigned long nr_running(void) > > > > /* > > * Check if only the current task is running on the cpu. > > + * > > + * Caution result is subject to time-of-check-to-time-of-use race, > > + * every caller is responsible to set up additional fences if necessary. > > Let's expand it a bit more: > > * Caution: this function does not check that the caller has disabled > * preemption, thus the result might have a time-of-check-to-time-of-use > * race. The caller is responsible to use this correctly, for example: > * > * - use it from a non-preemptable section > * > * - use it from a thread that is bound to a single CPU > * > * - use it in a loop where each iteration takes very little time > * (e.g. a polling loop) > */ > > I'll include it in my pull request. Sounds really good. Thank you! > Paolo > > > */ > > bool single_task_running(void) > > { > > - if (cpu_rq(smp_processor_id())->nr_running == 1) > > - return true; > > - else > > - return false; > > + return raw_rq()->nr_running == 1; > > } > > EXPORT_SYMBOL(single_task_running); > > > > >