From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759350Ab0FJPLu (ORCPT ); Thu, 10 Jun 2010 11:11:50 -0400 Received: from casper.infradead.org ([85.118.1.10]:43694 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753925Ab0FJPLt convert rfc822-to-8bit (ORCPT ); Thu, 10 Jun 2010 11:11:49 -0400 Subject: Re: BUG: sched:sched_migrate_task event tracing returns incorrect events From: Peter Zijlstra To: Ronny =?ISO-8859-1?Q?Tsch=FCter?= Cc: Ingo Molnar , Thomas Gleixner , Linux Kernel Mailing List In-Reply-To: <4C10FD1C.7080200@tu-dresden.de> References: <4C10FD1C.7080200@tu-dresden.de> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8BIT Date: Thu, 10 Jun 2010 17:11:41 +0200 Message-ID: <1276182701.2077.462.camel@twins> Mime-Version: 1.0 X-Mailer: Evolution 2.28.3 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 2010-06-10 at 16:56 +0200, Ronny Tschüter wrote: > Hi, > > I want to know a little bit more about the execution of my program. > Especially I'm interested in cpu migrations of my process. Therefore I > use the Linux performance counter infrastructure, set appropriate > perf_event attributes and setup the sched:sched_migrate_task event with > a system call: > ... > attr.type = PERF_TYPE_TRACEPOINT; > attr.size = sizeof( struct perf_event_attr ); > attr.config = event_id; // id of sched:sched_migrate_task event > attr.sample_type = PERF_SAMPLE_TIME|PERF_SAMPLE_CPU|PERF_SAMPLE_RAW; > attr.freq = 0; > attr.sample_freq = 1; > fd = syscall( __NR_perf_counter_open, &attr, getpid(), -1, -1, 0 ); > ... > Finally to read the events I map the kernel event buffer via mmap and > process every event in this buffer. However I run into two problems: > > (1) I need root privileges to open the counter successfully. Without > root privileges syscall returns -1. I think that is not OK because users > should be allowed to create per task counters. It is, because you specify PERF_SAMPLE_RAW, which is considered a data leak for unpriv users. > (2) The buffer contains events that aren't caused by my process (i.e. > the COMM field does not contain the name of my process and the PID field > is not equal to the pid of my process). I think that could happen if the migration happens from interrupt context while your task is running. Also wakeups your process does can result in migrations of the woken task from the context of your task.