From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751628AbaBCGTg (ORCPT ); Mon, 3 Feb 2014 01:19:36 -0500 Received: from mail-la0-f45.google.com ([209.85.215.45]:34244 "EHLO mail-la0-f45.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750784AbaBCGTf (ORCPT ); Mon, 3 Feb 2014 01:19:35 -0500 Date: Mon, 3 Feb 2014 10:19:29 +0400 From: Cyrill Gorcunov To: Don Zickus Cc: Peter Zijlstra , LKML Subject: Re: [PATCH] x86, perf, p4: Counter corruption when using lots of perf groups Message-ID: <20140203061929.GF1850@moon> References: <1391024270-19469-1-git-send-email-dzickus@redhat.com> <20140129200657.GJ29846@moon> <20140129201717.GB25953@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140129201717.GB25953@redhat.com> 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 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!