From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752834AbaBCQgi (ORCPT ); Mon, 3 Feb 2014 11:36:38 -0500 Received: from mx1.redhat.com ([209.132.183.28]:30243 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751999AbaBCQgf (ORCPT ); Mon, 3 Feb 2014 11:36:35 -0500 Date: Mon, 3 Feb 2014 11:35:30 -0500 From: Don Zickus To: Cyrill Gorcunov Cc: Peter Zijlstra , LKML Subject: Re: [PATCH] x86, perf, p4: Counter corruption when using lots of perf groups Message-ID: <20140203163530.GK25953@redhat.com> References: <1391024270-19469-1-git-send-email-dzickus@redhat.com> <20140129200657.GJ29846@moon> <20140129201717.GB25953@redhat.com> <20140203061929.GF1850@moon> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140203061929.GF1850@moon> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Feb 03, 2014 at 10:19:29AM +0400, Cyrill Gorcunov wrote: > On Wed, Jan 29, 2014 at 03:17:17PM -0500, Don Zickus wrote: > > > > I am not entirely sure on the corruption path, but what happens is: > > > > > > > > o perf schedules a group with p4_pmu_schedule_events() > > > > o inside p4_pmu_schedule_events(), it notices an hwc pointer is being reused > > > > but for a different cpu, so it 'swaps' the config bits and returns the > > > > updated 'assign' array with a _new_ index. > > > > o perf schedules another group with p4_pmu_schedule_events() > > > > o inside p4_pmu_schedule_events(), it notices an hwc pointer is being reused > > > > (the same one as above) but for the _same_ cpu [BUG!!], so it updates the > > > > 'assign' array to use the _old_ (wrong cpu) index because the _new_ index is in > > > > an earlier part of the 'assign' array (and hasn't been committed yet). > > > > o perf commits the transaction using the wrong index and corrupts the other cpu > > > > > > Thanks for the fix Don! I fear I won't be able to look precisely tonight, so > > > could it wait until tomorrow? (If it's critical sure such fix should do the > > > trick). > > > > There is no rush. Early next week is fine too. :-) > > Hi Don, sorry for delay. I thought maybe extending match_prev_assignment() > would be better (ie to figure out if previous event can run without > reprogramming the counter) but this makes code only harder (and what > is worse -- having no physical accees to p4 machine leaves no chance > to test changes). So eventually I think your patch does the same thing > as I had in mind but in different way. Thus > > Acked-by: Cyrill Gorcunov > > thanks a lot! thanks! Cheers, Don