From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758179AbZCZCnO (ORCPT ); Wed, 25 Mar 2009 22:43:14 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754961AbZCZCm6 (ORCPT ); Wed, 25 Mar 2009 22:42:58 -0400 Received: from bilbo.ozlabs.org ([203.10.76.25]:34150 "EHLO bilbo.ozlabs.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752238AbZCZCm5 (ORCPT ); Wed, 25 Mar 2009 22:42:57 -0400 MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Message-ID: <18890.59107.116394.771297@drongo.ozlabs.ibm.com> Date: Thu, 26 Mar 2009 13:22:27 +1100 From: Paul Mackerras To: Peter Zijlstra Cc: Ingo Molnar , linux-kernel@vger.kernel.org, Mike Galbraith , Arjan van de Ven , Wu Fengguang Subject: Re: [PATCH 5/6] perf_counter: kerneltop: mmap_pages argument In-Reply-To: <1237984033.7972.865.camel@twins> References: <20090325113021.781490788@chello.nl> <20090325113317.104545398@chello.nl> <20090325121811.GC11571@elte.hu> <1237984033.7972.865.camel@twins> X-Mailer: VM 8.0.12 under 22.2.1 (powerpc-unknown-linux-gnu) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Peter Zijlstra writes: > One possible solution is making the control page writable and writing > the userspace read position to it, then the kernel can, on > perf_output_begin() detect the overflow and count the number of > overwritten events. > > This overflow count could then be published back into the control page. We could in principle have many different processes mmapping the same counter and reading the ring buffer, couldn't we? So which process gets to put its read position in the control page? > TBH I'm not much of a fan, making all these pages writable just opens a > whole can of worms, and that accurate overflow tracking will put more > code in the output path. I agree. Paul.