From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932863Ab1C3TyT (ORCPT ); Wed, 30 Mar 2011 15:54:19 -0400 Received: from mx1.redhat.com ([209.132.183.28]:53615 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932331Ab1C3TyS (ORCPT ); Wed, 30 Mar 2011 15:54:18 -0400 Date: Wed, 30 Mar 2011 21:53:54 +0200 From: Oleg Nesterov To: "Paul E. McKenney" Cc: Peter Zijlstra , Jiri Olsa , Paul Mackerras , Ingo Molnar , linux-kernel@vger.kernel.org Subject: Re: [PATCH,RFC] perf: panic due to inclied cpu context task_ctx value Message-ID: <20110330195354.GA15046@redhat.com> References: <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> <20110330163730.GA6038@redhat.com> <20110330183049.GK2255@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20110330183049.GK2255@linux.vnet.ibm.com> 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, Paul E. McKenney wrote: > > On Wed, Mar 30, 2011 at 06:37:30PM +0200, Oleg Nesterov wrote: > > > > 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. > > For rcu_read_lock() in preemptible RCU, this is true. Hmm, not sure I understand... Do you mean that with the current implementation rcu_read_unlock() can imply rcu_sched_qs() without rescheduling ? > But for > rcu_read_unlock_sched(), the only way rcu_note_context_switch() is called > is if the code is preempted, which ends up calling schedule(). Indeed, that is why I thought synchronize_sched() can help in this case. I meant, according to the documentation it could in theory. But, > > Oh, and this is not theoretical, afaics. run_ksoftirqd() does > > rcu_note_context_switch(). > > Interesting... Color me confused. > > Suppose the rcu_note_context_switch() in run_ksoftirqd() was replaced > with schedule(). This has to be OK, right? But schedule() itself > invokes rcu_note_context_switch(). So if it is OK to call schedule(), > it should be OK to call rcu_note_context_switch() directly, right? > > So, what am I missing here? It is me, not you. Damn. It is even worse than I thought. Somehow I missed the simple fact, schedule() does not necessarily mean context_switch(). So the idea to use synchronize_sched() was simply wrong. Sorry to all for wasting your time ;) > > So, I think we need something else :/ > > The thing that I would be more concerned about is the idle loop. > If a CPU is in the idle loop, then rcu_sched_qs() will be invoked > (and which is invoked by rcu_note_context_switch()). So is it > illegal to use the above in the idle loop? Not sure I understand what you mean, but the idle loop is fine. An idle thread can't have the counters attached, we don't care about them. Thanks Paul, Oleg.