From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757304AbZEEHmc (ORCPT ); Tue, 5 May 2009 03:42:32 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752680AbZEEHmX (ORCPT ); Tue, 5 May 2009 03:42:23 -0400 Received: from mx2.mail.elte.hu ([157.181.151.9]:58499 "EHLO mx2.mail.elte.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752000AbZEEHmX (ORCPT ); Tue, 5 May 2009 03:42:23 -0400 Date: Tue, 5 May 2009 09:42:11 +0200 From: Ingo Molnar To: Peter Zijlstra Cc: Corey Ashford , linux-kernel@vger.kernel.org, Paul Mackerras Subject: Re: perf_counter: resetting a event counter Message-ID: <20090505074211.GA18437@elte.hu> References: <49FF68EF.40300@linux.vnet.ibm.com> <20090505065206.GB27191@elte.hu> <1241508860.11059.24.camel@twins> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1241508860.11059.24.camel@twins> User-Agent: Mutt/1.5.18 (2008-05-17) X-ELTE-VirusStatus: clean X-ELTE-SpamScore: -1.5 X-ELTE-SpamLevel: X-ELTE-SpamCheck: no X-ELTE-SpamVersion: ELTE 2.0 X-ELTE-SpamCheck-Details: score=-1.5 required=5.9 tests=BAYES_00 autolearn=no SpamAssassin version=3.2.3 -1.5 BAYES_00 BODY: Bayesian spam probability is 0 to 1% [score: 0.0000] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Peter Zijlstra wrote: > On Tue, 2009-05-05 at 08:52 +0200, Ingo Molnar wrote: > > * Corey Ashford wrote: > > > > > Hi, > > > > > > In implementing the PAPI_reset function, whose purpose is to reset > > > all of the counters in an event set, I found that [it appears] > > > there is no straight-forward way to implement this function using > > > "Performance Couunters for Linux". My current implementation just > > > closes the counters and reopens them again. This is not a elegant > > > solution, nor is the other alternative that occurred to me: > > > maintain a "virtual" counter in user space, maintained using a > > > base count, which is subtracted off of the current perf_counter > > > value of a particular counter. > > > > > > Is there a way that I missed to reset an event counter? If not, > > > I'd like to request that a new ioctl command be added to support > > > this ability. > > > > We already have such ioctl actions: > > > > case PERF_COUNTER_IOC_ENABLE: > > case PERF_COUNTER_IOC_DISABLE: > > case PERF_COUNTER_IOC_REFRESH: > > > > It would be a pretty natural addition to also have a reset method > > there. Would you like to take a stab at it and send a patch? A > > first-level approximation would be to do something like: > > > > perf_counter_disable(counter); > > atomic64_set(&counter->count, 0); > > perf_counter_enable(counter); > > > > btw, the reset code should probably take the counter->mutex lock as > > well, because parallel resets done from multiple contexts are > > otherwise not well-defined. > > I would suggest simply bailing when the counter is active when > trying to reset if anything. > > A plain: atomic64_set(&counter->count, 0), sounds attractive too. > > The trouble with putting in those disable/enable calls is that you > cannot use the ioctl on an already disabled call, since it will > immediately enable it. Also, using it on an active counter is racy > in nature so the disable/enable cycle (or the proposed mutex) > doesn't buy you anything. Yes, it all seems a bit racy - but straightforward. I dont think we should restrict the ioctl to disabled state alone - we should disable it if it's not disabled - reset the counter - then re-enable-it if it was enabled before. Btw., hw_counter->prev_counter needs to be reset too (if it's a hw counter not a sw counter), right? Ingo