From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752359AbdI1L3D (ORCPT ); Thu, 28 Sep 2017 07:29:03 -0400 Received: from foss.arm.com ([217.140.101.70]:55456 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751058AbdI1L3B (ORCPT ); Thu, 28 Sep 2017 07:29:01 -0400 Date: Thu, 28 Sep 2017 12:29:15 +0100 From: Will Deacon To: Alexander Shishkin Cc: Peter Zijlstra , Ingo Molnar , linux-kernel@vger.kernel.org Subject: Re: [PATCH] perf/aux: Only update aux_wakeup in non-overwrite mode Message-ID: <20170928112915.GB24145@arm.com> References: <20170906160811.16510-1-alexander.shishkin@linux.intel.com> <20170912223732.GD10675@arm.com> <87fubqx4ys.fsf@ashishki-desk.ger.corp.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87fubqx4ys.fsf@ashishki-desk.ger.corp.intel.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Alexander, On Wed, Sep 13, 2017 at 01:34:03PM +0300, Alexander Shishkin wrote: > Will Deacon writes: > > On Wed, Sep 06, 2017 at 07:08:11PM +0300, Alexander Shishkin wrote: > >> Commit d9a50b0256 ("perf/aux: Ensure aux_wakeup represents most recent > >> wakeup index") changed aux wakeup position calculation to rounddown(), > >> which causes a division-by-zero in AUX overwrite mode (aka "snapshot > >> mode"). > >> > >> The zero denominator results from the fact that perf record doesn't set > >> aux_watermark to anything, in which case the kernel will set it to half > >> the AUX buffer size, but only for non-overwrite mode. In the overwrite > >> mode aux_watermark stays zero. > >> > >> The good news is that, AUX overwrite mode, wakeups don't happen and > >> related bookkeeping is not relevant, so we can simply forego the whole > >> wakeup updates. > >> > >> Signed-off-by: Alexander Shishkin > >> --- > >> kernel/events/ring_buffer.c | 20 +++++++++++++++----- > >> 1 file changed, 15 insertions(+), 5 deletions(-) > > > > Damn, sorry about that. How did you spot the problem? > > A normal perf record with -S should trigger it, I don't remember what > exactly I was doing at that moment. But no worries, we all missed it the > first time around. :) > > > Anyway, I think the code is much better with this factored out: > > > > Acked-by: Will Deacon > > Thanks! What's the status on this patch? I don't see it in mainline or linux-next, but it would ideally get in as a fix for 4.14. Will