From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757227AbZEVJ4r (ORCPT ); Fri, 22 May 2009 05:56:47 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753924AbZEVJ4Z (ORCPT ); Fri, 22 May 2009 05:56:25 -0400 Received: from bilbo.ozlabs.org ([203.10.76.25]:33101 "EHLO bilbo.ozlabs.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752066AbZEVJ4Y (ORCPT ); Fri, 22 May 2009 05:56:24 -0400 MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Message-ID: <18966.30400.611789.434936@cargo.ozlabs.ibm.com> Date: Fri, 22 May 2009 19:56:16 +1000 From: Paul Mackerras To: Peter Zijlstra Cc: Ingo Molnar , linux-kernel@vger.kernel.org, Corey Ashford , Thomas Gleixner Subject: Re: [PATCH 2/2] perf_counter: optimize context switch between identical inherited contexts In-Reply-To: <1242980169.26820.617.camel@twins> References: <18966.10075.781053.231153@cargo.ozlabs.ibm.com> <18966.10666.517218.332164@cargo.ozlabs.ibm.com> <1242980169.26820.617.camel@twins> 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 Peter Zijlstra writes: > On Fri, 2009-05-22 at 14:27 +1000, Paul Mackerras wrote: > > Since we don't have individual fds for the counters in a cloned > > context, the only thing that can make two clones of a given parent > > different after they have been cloned is enabling or disabling all > > counters with prctl. To account for this, we keep a count of the > > number of enabled counters in each context. Two contexts must have > > the same number of enabled counters to be considered equivalent. > > Curious point that.. so prctl() can disable counters it doesn't own. > > Shouldn't we instead fix that? Well, prctl enables/disables the counters that are counting on the current process, regardless of who or what created them. I always thought that was a little strange; maybe it is useful to be able to disable all the counters that any other process might have put on to you, but I can't think of a scenario where you'd really need to do that, particularly since the disable is a one-shot operation, and doesn't prevent new (enabled) counters being attached to you. On the other hand, what does "all the counters I own" mean? Does it mean all the ones that I have fds open for? Or does it mean all the ones that I created? Either way we don't have a good way to enumerate them. Paul.