From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751678AbdILWhY (ORCPT ); Tue, 12 Sep 2017 18:37:24 -0400 Received: from foss.arm.com ([217.140.101.70]:46126 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750984AbdILWhX (ORCPT ); Tue, 12 Sep 2017 18:37:23 -0400 Date: Tue, 12 Sep 2017 23:37:32 +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: <20170912223732.GD10675@arm.com> References: <20170906160811.16510-1-alexander.shishkin@linux.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170906160811.16510-1-alexander.shishkin@linux.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 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? Anyway, I think the code is much better with this factored out: Acked-by: Will Deacon Thanks, Will > diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c > index f4ebe42879..4dae1da4c1 100644 > --- a/kernel/events/ring_buffer.c > +++ b/kernel/events/ring_buffer.c > @@ -412,6 +412,19 @@ void *perf_aux_output_begin(struct perf_output_handle *handle, > return NULL; > } > > +static bool __always_inline rb_need_aux_wakeup(struct ring_buffer *rb) > +{ > + if (rb->aux_overwrite) > + return false; > + > + if (rb->aux_head - rb->aux_wakeup >= rb->aux_watermark) { > + rb->aux_wakeup = rounddown(rb->aux_head, rb->aux_watermark); > + return true; > + } > + > + return false; > +} > + > /* > * Commit the data written by hardware into the ring buffer by adjusting > * aux_head and posting a PERF_RECORD_AUX into the perf buffer. It is the > @@ -451,10 +464,8 @@ void perf_aux_output_end(struct perf_output_handle *handle, unsigned long size) > } > > rb->user_page->aux_head = rb->aux_head; > - if (rb->aux_head - rb->aux_wakeup >= rb->aux_watermark) { > + if (rb_need_aux_wakeup(rb)) > wakeup = true; > - rb->aux_wakeup = rounddown(rb->aux_head, rb->aux_watermark); > - } > > if (wakeup) { > if (handle->aux_flags & PERF_AUX_FLAG_TRUNCATED) > @@ -484,9 +495,8 @@ int perf_aux_output_skip(struct perf_output_handle *handle, unsigned long size) > rb->aux_head += size; > > rb->user_page->aux_head = rb->aux_head; > - if (rb->aux_head - rb->aux_wakeup >= rb->aux_watermark) { > + if (rb_need_aux_wakeup(rb)) { > perf_output_wakeup(handle); > - rb->aux_wakeup = rounddown(rb->aux_head, rb->aux_watermark); > handle->wakeup = rb->aux_wakeup + rb->aux_watermark; > } > > -- > 2.14.1 >