From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753305Ab0CBOSh (ORCPT ); Tue, 2 Mar 2010 09:18:37 -0500 Received: from tx2ehsobe005.messaging.microsoft.com ([65.55.88.15]:49908 "EHLO TX2EHSOBE010.bigfish.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751352Ab0CBOSg (ORCPT ); Tue, 2 Mar 2010 09:18:36 -0500 X-SpamScore: -24 X-BigFish: VPS-24(zz1432R98dN936eM62a3Lzz1202hzzz32i2a8h6bh61h) X-Spam-TCS-SCL: 0:0 X-WSS-ID: 0KYNRQO-02-53B-02 X-M-MSG: Date: Tue, 2 Mar 2010 15:18:24 +0100 From: Robert Richter To: Peter Zijlstra CC: eranian@google.com, linux-kernel@vger.kernel.org, mingo@elte.hu, paulus@samba.org, fweisbec@gmail.com, perfmon2-devel@lists.sf.net, eranian@gmail.com Subject: Re: [PATCH] perf_events: add sampling period randomization support Message-ID: <20100302141824.GG13205@erda.amd.com> References: <1267510029-19750-1-git-send-email-eranian@google.com> <20100302105314.GF13205@erda.amd.com> <1267530078.25158.41.camel@laptop> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Disposition: inline In-Reply-To: <1267530078.25158.41.camel@laptop> User-Agent: Mutt/1.5.20 (2009-06-14) X-OriginalArrivalTime: 02 Mar 2010 14:18:25.0059 (UTC) FILETIME=[389F4F30:01CABA13] X-Reverse-DNS: ausb3extmailp02.amd.com Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 02.03.10 12:41:18, Peter Zijlstra wrote: > On Tue, 2010-03-02 at 11:53 +0100, Robert Richter wrote: > > > > Only adding the random value will lead to longer sample periods on > > average. To compensate this you could calculate something like: > > > > event->hw.sample_period = event->attr.sample_period + (new_seed & mask) - (mask >> 1); > > Or cheat and do something like: > > sample_period ^= (new_seed & mask); This wont work, it will be asymmetric, e.g. for (event->attr.sample_period & mask) == 0 the offset would be always positive. Only for (event->attr.sample_period & mask) == (mask & ~(mask >> 1)) it is correct. -Robert -- Advanced Micro Devices, Inc. Operating System Research Center email: robert.richter@amd.com