From mboxrd@z Thu Jan 1 00:00:00 1970 From: Paolo Bonzini Subject: Re: [PATCH] sched: access local runqueue directly in single_task_running Date: Fri, 18 Sep 2015 13:26:53 +0200 Message-ID: <55FBF4FD.6060609@redhat.com> References: <1442568465-71559-1-git-send-email-dingel@linux.vnet.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Ingo Molnar , Tim Chen , stable@vger.kernel.org To: Dominik Dingel , Peter Zijlstra Return-path: In-Reply-To: <1442568465-71559-1-git-send-email-dingel@linux.vnet.ibm.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org 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. 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); > >