From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752732AbaIOOXZ (ORCPT ); Mon, 15 Sep 2014 10:23:25 -0400 Received: from mail-pa0-f50.google.com ([209.85.220.50]:44881 "EHLO mail-pa0-f50.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752144AbaIOOXY (ORCPT ); Mon, 15 Sep 2014 10:23:24 -0400 Message-ID: <5416F658.5090105@gmail.com> Date: Mon, 15 Sep 2014 08:23:20 -0600 From: David Ahern User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.9; rv:24.0) Gecko/20100101 Thunderbird/24.6.0 MIME-Version: 1.0 To: Alexander Yarygin CC: linux-kernel@vger.kernel.org, Arnaldo Carvalho de Melo , Christian Borntraeger , Ingo Molnar , Jiri Olsa , Paul Mackerras , Peter Zijlstra Subject: Re: [PATCH RFC] perf kvm stat live: cache mmap()ed events References: <1410539263-25857-1-git-send-email-yarygin@linux.vnet.ibm.com> <5415B6B9.2040607@gmail.com> <874mw9vy69.fsf@linux.vnet.ibm.com> In-Reply-To: <874mw9vy69.fsf@linux.vnet.ibm.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 9/15/14, 6:57 AM, Alexander Yarygin wrote: > David Ahern writes: > >> On 9/12/14, 10:27 AM, Alexander Yarygin wrote: >>> During mmap() process 'perf kvm stat live' gets a pointer to events and >>> passes them to the session queue. Events are stored in shared memory and >>> eventually they will be overwritten by the kernel. The problem is, that >>> when events come too fast, old events can be overwritten before they >>> have been processed that can lead to perf crash. >>> >>> To prevent that happening, we can copy upcoming events and pass a copy >>> to the session queue. There is a safe place to copy event: before >>> perf_evlist__mmap_consume() is executed. There are 3 places to free it: >>> when event is processed, when it's lost and on exit, if it's turned out >>> unprocessed. >> >> Did you see what I proposed a year ago: >> https://lkml.org/lkml/2013/9/6/388 >> >> The intent is to keep the copy generic and not local a command since >> conceptually other live commands need the same. >> >> David > > Hello, > > Yes, your patch works fine. But as far as I understand, right now only > the 'perf kvm stat live' is infected: other 'live' tools were fixed by > the patch "PERF: The tail position of the event buffer should only be modified > after actually use that event." and since they don't use ordered queue > they don't need a copying. That's why I came up with 'perf kvm stat > live' specific approach. Maybe I missed something... No, you understand the problem. My point is that the key issue with the code (overwriting events in the mmap'ed buffers) really has nothing to do with perf-kvm. It might be the only command in-tree today but others could come along, so might as well fix the issue in the event processing code. > > Anyhow, having >30K events is a quite usual situation on s390 and the > 'perf kvm stat live' command hardly works there, so it would be good to > have at least some working solution. Any ideas? :) Sure. With nested VMs on x86 I see 100,000's of events per second. That use case is what drove me to look at copying events. I just have not had time to come back to that one. David