Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1760472AbYJIRyf (ORCPT ); Thu, 9 Oct 2008 13:54:35 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1757167AbYJIRy0 (ORCPT ); Thu, 9 Oct 2008 13:54:26 -0400 Received: from e5.ny.us.ibm.com ([32.97.182.145]:40137 "EHLO e5.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754944AbYJIRyY (ORCPT ); Thu, 9 Oct 2008 13:54:24 -0400 Subject: Re: [PATCH] sched_clock: prevent scd->clock from moving backwards From: Dave Kleikamp To: Ingo Molnar Cc: Peter Zijlstra , Jeremy Fitzhardinge , Steven Rostedt , Linux Kernel Mailing List In-Reply-To: <20081009151703.GA8010@elte.hu> References: <48D959E8.4000303@goop.org> <1223470773.6336.13.camel@norville.austin.ibm.com> <1223470854.6336.15.camel@norville.austin.ibm.com> <1223507104.7382.6.camel@lappy.programming.kicks-ass.net> <20081009090605.GA21798@elte.hu> <20081009151703.GA8010@elte.hu> Content-Type: text/plain Date: Thu, 09 Oct 2008 12:54:22 -0500 Message-Id: <1223574862.6407.16.camel@norville.austin.ibm.com> Mime-Version: 1.0 X-Mailer: Evolution 2.22.3.1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1944 Lines: 55 On Thu, 2008-10-09 at 17:17 +0200, Ingo Molnar wrote: > hm, -tip testing found a sporadic hard lockup during bootup, and i've > bisected it back to this patch. They happened on 64-bit test-systems. > I've attached the .config that produced the problem. > > i reverted the patch and the lockups went away. But i cannot see what's > wrong with it ... I could have sworn I ran with the patch, but maybe I got my patch queue messed up and never tested it right. I think I see the problem. --- a/kernel/sched_clock.c +++ b/kernel/sched_clock.c @@ -118,13 +118,13 @@ static u64 __update_sched_clock(struct sched_clock_data *scd, u64 now) /* * scd->clock = clamp(scd->tick_gtod + delta, - * max(scd->tick_gtod, scd->clock), - * scd->tick_gtod + TICK_NSEC); + * max(scd->tick_gtod, scd->clock), + * min(scd->clock, scd->tick_gtod + TICK_NSEC)); */ clock = scd->tick_gtod + delta; min_clock = wrap_max(scd->tick_gtod, scd->clock); - max_clock = scd->tick_gtod + TICK_NSEC; + max_clock = wrap_min(scd->clock, scd->tick_gtod + TICK_NSEC); clock = wrap_max(clock, min_clock); clock = wrap_min(clock, max_clock); We want wrap_max(scd->clock, scd->tick_gtod + TICK_NSEC), not wrap_min(). The problem I am trying to fix is that scd->tick_gtod + TICK_NSEC may be too low. The upper bound needs to be at LEAST scd->clock. Limiting it to scd->clock all the time is disastrous. :-) I'll fix the patch and retest it before sending it again. Sorry about my sloppiness. Shaggy -- David Kleikamp IBM Linux Technology Center -- 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/