Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755358Ab3JBSO2 (ORCPT ); Wed, 2 Oct 2013 14:14:28 -0400 Received: from mail-qe0-f48.google.com ([209.85.128.48]:45608 "EHLO mail-qe0-f48.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753855Ab3JBSO1 (ORCPT ); Wed, 2 Oct 2013 14:14:27 -0400 MIME-Version: 1.0 In-Reply-To: <524C5B10.20006@codeaurora.org> References: <1380732928-13897-1-git-send-email-santosh.shilimkar@ti.com> <20131002170917.GB30298@mudshark.cambridge.arm.com> <524C5484.5000601@ti.com> <524C565C.8010709@codeaurora.org> <524C5786.2090008@ti.com> <524C5B10.20006@codeaurora.org> Date: Wed, 2 Oct 2013 13:14:26 -0500 Message-ID: Subject: Re: [PATCH] sched_clock: fix postinit no sched_clock function check From: Rob Herring To: Stephen Boyd Cc: Santosh Shilimkar , Russell King , Daniel Lezcano , Will Deacon , "linux-kernel@vger.kernel.org" , "rob.herring@calxeda.com" , "arm@kernel.org" , John Stultz , Thomas Gleixner , "linux-arm-kernel@lists.infradead.org" Content-Type: text/plain; charset=ISO-8859-1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2812 Lines: 51 On Wed, Oct 2, 2013 at 12:42 PM, Stephen Boyd wrote: > On 10/02/13 10:27, Santosh Shilimkar wrote: >> On Wednesday 02 October 2013 01:22 PM, Stephen Boyd wrote: >>> On 10/02/13 10:14, Santosh Shilimkar wrote: >>>> On Wednesday 02 October 2013 01:09 PM, Will Deacon wrote: >>>>> On Wed, Oct 02, 2013 at 05:55:28PM +0100, Santosh Shilimkar wrote: >>>>>> The sched_clock code uses 2 levels of function pointers, sched_clock_func() >>>>>> and read_sched_clock() but the no sched_clock check in postinit() just >>>>>> checks read_sched_clock(). >>>>>> >>>>>> This leads to kernel falling back to jiffy based sched clock even in >>>>>> presence of sched_clock_func() which is not desirable. >>>>>> >>>>>> Fix the postinit() check to avoid the issue. Probably the issue is hidden >>>>>> so far on most of the arm SOCs because of already existing sched_clock >>>>>> registrations apart from arch_timer sched_clock. One can reproduce the >>>>>> issue by just have arch_timer as sched_clock >>>>> Isn't this just an issue with the arch timer driver not calling >>>>> setup_sched_clock? Instead, we munge around with sched_clock_func directly, >>>>> which doesn't appear to be the way anybody else deals with this. >>>>> >>>> I thought about that option as well but was not sure since even in that case >>>> the check is not complete. We just ensure that function is popullated. >>> Yes, nothing is actually broken because sched_clock_func() won't try to >>> use the jiffy based read_sched_clock() function. I'm not sure we >>> actually need this patch besides to remove a useless timer that updates >>> the jiffy epoch. Can we wait until my 64-bit sched_clock patch series >>> lands in 3.13? It looks like I still need an ack from Will or Catalin on >>> the architected timer patch before the clocksource folks pick it up. >>> >> Really... I have not created patch out of fun. >> Its broken on my keystone machine at least where the sched_clock is >> falling back on jiffy based sched_clock even in presence of arch_timer >> sched_clock. > > How is that possible? sched_clock_func is only assigned by > arch/arm/kernel/arch_timer.c when the architected timer is detected and > sched_clock() in kernel/time/sched_clock.c calls that function pointer > unconditionally. The only way I see this happening is if the architected > timer rate is zero. I agree we will get two lines in the dmesg about > sched_clock and its not very clear which one is being used. It does setup a timer to run to handle wrapping which should be harmless, but isn't needed. Rob -- 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/