From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759318AbZE1IpX (ORCPT ); Thu, 28 May 2009 04:45:23 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754823AbZE1IpF (ORCPT ); Thu, 28 May 2009 04:45:05 -0400 Received: from 124x34x33x190.ap124.ftth.ucom.ne.jp ([124.34.33.190]:50541 "EHLO master.linux-sh.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754058AbZE1IpB (ORCPT ); Thu, 28 May 2009 04:45:01 -0400 Date: Thu, 28 May 2009 17:44:44 +0900 From: Paul Mundt To: Daniel Walker Cc: Thomas Gleixner , Peter Zijlstra , Linus Walleij , Ingo Molnar , Andrew Victor , Haavard Skinnemoen , Andrew Morton , linux-kernel@vger.kernel.org, linux-sh@vger.kernel.org, linux-arm-kernel@lists.arm.linux.org.uk, John Stultz Subject: Re: [PATCH] sched: Support current clocksource handling in fallback sched_clock(). Message-ID: <20090528084444.GA26955@linux-sh.org> Mail-Followup-To: Paul Mundt , Daniel Walker , Thomas Gleixner , Peter Zijlstra , Linus Walleij , Ingo Molnar , Andrew Victor , Haavard Skinnemoen , Andrew Morton , linux-kernel@vger.kernel.org, linux-sh@vger.kernel.org, linux-arm-kernel@lists.arm.linux.org.uk, John Stultz References: <20090526061532.GD9188@linux-sh.org> <63386a3d0905260731m655bfee3q82a6f52d71fa3cef@mail.gmail.com> <1243348681.23657.14.camel@twins> <20090526230855.GA27218@linux-sh.org> <20090527001543.GA8493@linux-sh.org> <1243441525.28705.19.camel@desktop> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1243441525.28705.19.camel@desktop> User-Agent: Mutt/1.5.13 (2006-08-11) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, May 27, 2009 at 09:25:25AM -0700, Daniel Walker wrote: > On Wed, 2009-05-27 at 09:15 +0900, Paul Mundt wrote: > > > unsigned long long __attribute__((weak)) sched_clock(void) > > { > > + /* > > + * Use the current clocksource when it becomes available later in > > + * the boot process, and ensure that it is usable for sched_clock(). > > + */ > > + if (clock && (clock->flags & CLOCK_SOURCE_USE_FOR_SCHED_CLOCK)) > > + return cyc2ns(clock, clocksource_read(clock)); > > + > > + /* Otherwise just fall back on jiffies */ > > return (unsigned long long)(jiffies - INITIAL_JIFFIES) > > Do we really need all this complexity in a fast path? the jiffies > clocksource is static and always ready. Could we instead remove the > usage of "clock" and create a new pointer called "sched_clocksource" and > initialize it to the jiffies clock, and allow the clocksource management > code to update that new pointer based on the > CLOCK_SOURCE_USE_FOR_SCHED_CLOCK flag when new clocksources are > registered/removed/marked unstable etc.. > > that would eliminate all the code in sched_clock except one line, > > unsigned long long __attribute__((weak)) sched_clock(void) > { > return cyc2ns(sched_clocksource, clocksource_read(sched_clocksource)); > } > It would, but then what would that do to the sched_clock() value? It will reset once a CLOCK_SOURCE_USE_FOR_SCHED_CLOCK clocksource comes along, which basically means that sched_clock() then rolls back, which is worse than simply being delayed a bit. So even if we were to use a special sched_clocksource, it still could not be used until it was set up at registration time. Now we could of course add a sched_clocksource assignment in select_clocksource(), but that's really no different than just testing 'clock' as we do today. All it does is move the flag test in to a different path. So, I think the current v3 is still the best solution.