From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932738Ab1C3Qh4 (ORCPT ); Wed, 30 Mar 2011 12:37:56 -0400 Received: from mx1.redhat.com ([209.132.183.28]:60835 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753234Ab1C3Qhy (ORCPT ); Wed, 30 Mar 2011 12:37:54 -0400 Date: Wed, 30 Mar 2011 18:37:30 +0200 From: Oleg Nesterov To: Peter Zijlstra Cc: Jiri Olsa , Paul Mackerras , Ingo Molnar , linux-kernel@vger.kernel.org, "Paul E. McKenney" Subject: Re: [PATCH,RFC] perf: panic due to inclied cpu context task_ctx value Message-ID: <20110330163730.GA6038@redhat.com> References: <1301157483.2250.366.camel@laptop> <20110326170922.GA20329@redhat.com> <20110326173545.GA22919@redhat.com> <1301164168.2250.370.camel@laptop> <20110328133033.GA8254@redhat.com> <1301324275.4859.25.camel@twins> <1301327368.4859.28.camel@twins> <20110328165648.GA9304@redhat.com> <20110330130951.GA2124@jolsa.brq.redhat.com> <1301496684.4859.192.camel@twins> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1301496684.4859.192.camel@twins> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 03/30, Peter Zijlstra wrote: > > --- linux-2.6.orig/kernel/perf_event.c > +++ linux-2.6/kernel/perf_event.c > @@ -125,9 +125,25 @@ enum event_type_t { > * perf_sched_events : >0 events exist > * perf_cgroup_events: >0 per-cpu cgroup events exist on this cpu > */ > -atomic_t perf_sched_events __read_mostly; > +atomic_t perf_sched_events_in __read_mostly; > +atomic_t perf_sched_events_out __read_mostly; > static DEFINE_PER_CPU(atomic_t, perf_cgroup_events); > > +static void perf_sched_events_inc(void) > +{ > + jump_label_inc(&perf_sched_events_out); > + jump_label_inc(&perf_sched_events_in); > +} > + > +static void perf_sched_events_dec(void) > +{ > + jump_label_dec(&perf_sched_events_in); > + JUMP_LABEL(&perf_sched_events_in, no_sync); > + synchronize_sched(); > +no_sync: > + jump_label_dec(&perf_sched_events_out); > +} Nice! I didn't realize we can simply use JUMP_LABEL() directly and then the code doesn't depend on HAVE_JUMP_LABEL. Now, the problem is, after I read the comments I am not sure I understand what synchronize_sched() actually doe. Add Paul. So. synchronize_sched() above should ensure that all CPUs do context switch at least once (ignoring idle). And I _thought_ that in practice this should work. But, unles I misread the comment above synchronize_sched(), it seems that it only guarantees the end of "everything" which disables preemption, explicitly or not. IOW, say, in theory rcu_read_unlock_sched() could trigger ->passed_quiesc == T without reschedule. Oh, and this is not theoretical, afaics. run_ksoftirqd() does rcu_note_context_switch(). So, I think we need something else :/ Oleg.