From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751101AbVIVH2t (ORCPT ); Thu, 22 Sep 2005 03:28:49 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751431AbVIVH2t (ORCPT ); Thu, 22 Sep 2005 03:28:49 -0400 Received: from [85.8.12.41] ([85.8.12.41]:5507 "EHLO smtp.drzeus.cx") by vger.kernel.org with ESMTP id S1751101AbVIVH2t (ORCPT ); Thu, 22 Sep 2005 03:28:49 -0400 Message-ID: <43325D31.3090604@drzeus.cx> Date: Thu, 22 Sep 2005 09:28:49 +0200 From: Pierre Ossman User-Agent: Mozilla Thunderbird 1.0.6-5 (X11/20050818) X-Accept-Language: en-us, en MIME-Version: 1.0 To: LKML CC: Jesper Juhl , Dominik Brodowski Subject: [Fwd: sched_clock() has poor resolution] X-Enigmail-Version: 0.90.1.0 X-Enigmail-Supports: pgp-inline, pgp-mime Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org cc:ing some timer hackers. -------- Original Message -------- Subject: sched_clock() has poor resolution Date: Tue, 20 Sep 2005 12:02:34 +0200 From: Pierre Ossman To: LKML I've been trying to hunt down a HZ bug and as part of that I thought having printk timestamps would be nice. But too much dismay I noticed that I couldn't get very good resolution out of these. The problem turned out to be that sched_clock() uses jiffies as a time source. It will only use the TSC if that's the primary time source of the system. But since I have a PM timer that is given priority. So I see two solutions here: * Let sched_clock() follow the time source. Don't have it coupled to the TSC. * Init the TSC even though it isn't used for anything but sched_clock(). The first solution might have problems if one of the "better" timers are slow as hell to read and the second if there is some assumed dependency between sched_clock() and the primary timer. I'm blisfully ignorant of this area so input is welcome. Rgds Pierre - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/