From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759650Ab2ILOsb (ORCPT ); Wed, 12 Sep 2012 10:48:31 -0400 Received: from [205.233.59.134] ([205.233.59.134]:33567 "EHLO merlin.infradead.org" rhost-flags-FAIL-FAIL-OK-OK) by vger.kernel.org with ESMTP id S1759625Ab2ILOs2 convert rfc822-to-8bit (ORCPT ); Wed, 12 Sep 2012 10:48:28 -0400 Message-ID: <1347461232.15764.45.camel@twins> Subject: Re: [BUG] perf: multiplexing and hotplug CPU problem From: Peter Zijlstra To: Stephane Eranian Cc: LKML , "mingo@elte.hu" , "ak@linux.intel.com" , Robert Richter Date: Wed, 12 Sep 2012 16:47:12 +0200 In-Reply-To: References: Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7BIT X-Mailer: Evolution 3.2.2- Mime-Version: 1.0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 2012-09-12 at 16:40 +0200, Stephane Eranian wrote: > Hi, > > As I was debugging my hrtimer patch, I ran a few tests > with hotplug CPU. In others words, I offline a CPU while > there is an active monitoring session which causes multiplexing. > > When the CPU goes down, all is well. But when it comes back, > things go wrong. No kernel crashes but wrong results and multiplexing > does not work anymore. > > I investigated this some more and found out there is an issue > on re-activation. > > During shutdown, system-wide events are scheduled out AND removed > from the event lists. Consequently, ctx->nr_events and ctx->nr_active > go to zero. > > When the CPU is brought back online and tools do start/stop on the events > they can be scheduled back in, and therefore increment ctx->nr_active. > Because list_add_event() is not called again, you may end up with > ctx->nr_events < ctx->nr_active which is wrong. Events may not > be a lists and therefore they cannot get multiplexed again. > > It is not clear to me why we need to remove the events from any > list (list_del_event) when the CPU goes down. > > Why isn't calling event_sched_out() enough? > If events are kept on lists, why not try to schedule them back in when > the CPU is brought back online? This might be never, I think we should put the events in error state instead of disabling them, that should avoid the re-activation and provide a stronger hint to userspace that something went funny.