From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751407AbaJCGvT (ORCPT ); Fri, 3 Oct 2014 02:51:19 -0400 Received: from mx1.redhat.com ([209.132.183.28]:13552 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751282AbaJCGvS (ORCPT ); Fri, 3 Oct 2014 02:51:18 -0400 Date: Fri, 3 Oct 2014 08:50:47 +0200 From: Jiri Olsa To: Ingo Molnar Cc: Alexander Yarygin , linux-kernel@vger.kernel.org, Arnaldo Carvalho de Melo , Christian Borntraeger , David Ahern , Frederic Weisbecker , Mike Galbraith , Namhyung Kim , Paul Mackerras , Peter Zijlstra , Stephane Eranian Subject: Re: [PATCH 1/2] perf tools: Add option to copy events when queueing Message-ID: <20141003065047.GN9764@krava.brq.redhat.com> References: <1412267936-19827-1-git-send-email-yarygin@linux.vnet.ibm.com> <1412267936-19827-2-git-send-email-yarygin@linux.vnet.ibm.com> <20141003043421.GA24217@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20141003043421.GA24217@gmail.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 On Fri, Oct 03, 2014 at 06:34:21AM +0200, Ingo Molnar wrote: > > * Alexander Yarygin wrote: > > > When processing events the session code has an ordered samples > > queue which is used to time-sort events coming in across > > multiple mmaps. At a later point in time samples on the queue > > are flushed up to some timestamp at which point the event is > > actually processed. > > > > When analyzing events live (ie., record/analysis path in the > > same command) there is a race that leads to corrupted events > > and parse errors which cause perf to terminate. The problem is > > that when the event is placed in the ordered samples queue it > > is only a reference to the event which is really sitting in the > > mmap buffer. Even though the event is queued for later > > processing the mmap tail pointer is updated which indicates to > > the kernel that the event has been processed. The race is > > flushing the event from the queue before it gets overwritten by > > some other event. For commands trying to process events live > > (versus just writing to a file) and processing a high rate of > > events this leads to parse failures and perf terminates. > > > > Examples hitting this problem are 'perf kvm stat live', > > especially with nested VMs which generate 100,000+ traces per > > second, and a command processing scheduling events with a high > > rate of context switching -- e.g., running 'perf bench sched > > pipe'. > > > > This patch offers live commands an option to copy the event > > when it is placed in the ordered samples queue. > > What's the performance effect of this - i.e. by how much does CPU > use increase due to copying the events? > > Wouldn't it be faster to fix this problem by updating the mmap > tail pointer only once the event has truly been consumed? Alexander mentioned he'd loose data, because of userspace processing being to slow: http://marc.info/?l=linux-kernel&m=141111652424818&w=2 jirka