From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932389Ab0EYJr1 (ORCPT ); Tue, 25 May 2010 05:47:27 -0400 Received: from casper.infradead.org ([85.118.1.10]:48893 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932302Ab0EYJrZ convert rfc822-to-8bit (ORCPT ); Tue, 25 May 2010 05:47:25 -0400 Subject: Re: BUG: using smp_processor_id() in preemptible [00000000] code: icedove-bin/5449 From: Peter Zijlstra To: Ingo Molnar Cc: piotr@hosowicz.com, linux-kernel@vger.kernel.org, Jens Axboe , Divyesh Shah , Andrew Morton In-Reply-To: <20100525094347.GA7881@elte.hu> References: <4BF9EC69.5030709@example.com> <1274777422.5882.591.camel@twins> <20100525094347.GA7881@elte.hu> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8BIT Date: Tue, 25 May 2010 11:47:14 +0200 Message-ID: <1274780834.5882.730.camel@twins> Mime-Version: 1.0 X-Mailer: Evolution 2.28.3 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 2010-05-25 at 11:43 +0200, Ingo Molnar wrote: > > Subject: sched_clock: Add local_clock() > > From: Peter Zijlstra > > Date: Tue May 25 10:48:51 CEST 2010 > > > > For people who otherwise get to write: cpu_clock(smp_processor_id()), > > there is now: local_clock(). > > This doesnt fix the whole issue. cpu_clock() is local, while the measurements > done in the blk code are global ... > > While the warning is fixed this way, the far more serious issue is still > there: time can go backwards if two points of time measurement are on > different CPUs and can mess up the statistics with negative values, etc... cpu_clock() is synced on each tick, so the inter-cpu-drift should not exceed 2 jiffies. But yeah, if they want anything better, they'll have to start caring on what cpu which timestamp got taken and use cpu_clock().