From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756609AbZETXPl (ORCPT ); Wed, 20 May 2009 19:15:41 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753548AbZETXPd (ORCPT ); Wed, 20 May 2009 19:15:33 -0400 Received: from bilbo.ozlabs.org ([203.10.76.25]:36048 "EHLO bilbo.ozlabs.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752737AbZETXPd (ORCPT ); Wed, 20 May 2009 19:15:33 -0400 MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Message-ID: <18964.36622.94267.309523@cargo.ozlabs.ibm.com> Date: Thu, 21 May 2009 09:15:26 +1000 From: Paul Mackerras To: Ingo Molnar Cc: Peter Zijlstra , linux-kernel@vger.kernel.org, Corey Ashford , Thomas Gleixner Subject: Re: [RFC PATCH] perf_counter: dynamically allocate tasks' perf_counter_context struct In-Reply-To: <20090520171207.GA16706@elte.hu> References: <18963.63352.489273.92145@cargo.ozlabs.ibm.com> <20090520171207.GA16706@elte.hu> X-Mailer: VM 8.0.12 under 22.2.1 (i486-pc-linux-gnu) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Ingo Molnar writes: > * Paul Mackerras wrote: > > > This replaces the struct perf_counter_context in the task_struct > > with a pointer to a dynamically allocated perf_counter_context > > struct. The main reason for doing is this is to allow us to > > transfer a perf_counter_context from one task to another when we > > do lazy PMU switching in a later patch. > > Hm, i'm not sure how far this gets us towards lazy PMU switching. > > In fact i'd say that the term "lazy PMU switching" is probably > misleading, we should use: "equivalent PMU context switching" or > instead. Yes, that's what I mean. As you say, we need to be able to detect when two tasks have equivalent contexts - that is, when their counters are all inherited from a common ancestor. My idea is that in that situation we simply swap the contexts: move the context of the outgoing task onto the incoming task, and give the incoming task's context to the outgoing task. With my patch, that involves simply swapping the pointers over and adjusting the task pointers in the two contexts. That means that all the counters get transferred over to the incoming task, so there is nothing in the PMU or the arch code that needs to changed or adjusted. The outgoing task still has a perfectly valid context, so it doesn't matter if it migrates to another CPU. The nice thing is that there is nothing special or unusual about the state after we have swapped the contexts - nothing that needs to be remembered or undone later. Hopefully I'll have an actual patch to show you later today. Paul.