From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755268AbbLKPfA (ORCPT ); Fri, 11 Dec 2015 10:35:00 -0500 Received: from bombadil.infradead.org ([198.137.202.9]:40932 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751862AbbLKPe6 (ORCPT ); Fri, 11 Dec 2015 10:34:58 -0500 Date: Fri, 11 Dec 2015 16:34:54 +0100 From: Peter Zijlstra To: Alexander Shishkin Cc: Ingo Molnar , linux-kernel@vger.kernel.org, vince@deater.net, eranian@google.com, Arnaldo Carvalho de Melo , Mathieu Poirier Subject: Re: [PATCH v0 3/5] perf: Introduce instruction trace filtering Message-ID: <20151211153454.GZ6356@twins.programming.kicks-ass.net> References: <1449840998-29902-1-git-send-email-alexander.shishkin@linux.intel.com> <1449840998-29902-4-git-send-email-alexander.shishkin@linux.intel.com> <20151211150011.GT6356@twins.programming.kicks-ass.net> <871tatc8vu.fsf@ashishki-desk.ger.corp.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <871tatc8vu.fsf@ashishki-desk.ger.corp.intel.com> User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Dec 11, 2015 at 05:17:25PM +0200, Alexander Shishkin wrote: > Peter Zijlstra writes: > > > On Fri, Dec 11, 2015 at 03:36:36PM +0200, Alexander Shishkin wrote: > >> +static int __perf_event_itrace_filters_setup(void *info) > >> +{ > >> + struct perf_event *event = info; > >> + int ret; > >> + > >> + if (READ_ONCE(event->state) != PERF_EVENT_STATE_ACTIVE) > >> + return -EAGAIN; > >> + > >> + /* matches smp_wmb() in event_sched_in() */ > >> + smp_rmb(); > >> + > >> + /* > >> + * There is a window with interrupts enabled before we get here, > >> + * so we need to check again lest we try to stop another cpu's event. > >> + */ > >> + if (READ_ONCE(event->oncpu) != smp_processor_id()) > >> + return -EAGAIN; > >> + > >> + event->pmu->stop(event, PERF_EF_UPDATE); > >> + rcu_read_lock(); > >> + ret = event->pmu->itrace_filter_setup(event); > >> + rcu_read_unlock(); > >> + event->pmu->start(event, PERF_EF_RELOAD); > > > > Would it not be more sensible to let the ::itrace_filter_setup() method > > do the stop/start-ing if and when needed? > > I don't have a strong opinion on this, the only question is, are we > comfortable with pmu driver callback doing the > rcu_read_lock/unlock, because it still needs to iterate the filter list. > Other than that it's probably a good idea. See another email; I'm not sure RCU works for this. You can observe more events than you set out for, and if you do multiple iterations of the list they need not match.