Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759241AbZFBJAY (ORCPT ); Tue, 2 Jun 2009 05:00:24 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753882AbZFBJAN (ORCPT ); Tue, 2 Jun 2009 05:00:13 -0400 Received: from casper.infradead.org ([85.118.1.10]:33733 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754570AbZFBJAM (ORCPT ); Tue, 2 Jun 2009 05:00:12 -0400 Subject: Re: [PATCH] U300 sched_clock implementation From: Peter Zijlstra To: Linus Walleij Cc: Russell King - ARM Linux , linux-kernel@vger.kernel.org, mingo@elte.hu, linux-arm-kernel@lists.arm.linux.org.uk In-Reply-To: <63386a3d0906010046k3b86d70av3e290fc26c2934f5@mail.gmail.com> References: <63386a3d0905112337p2d426481o5f9bf9b9489cc57e@mail.gmail.com> <63386a3d0905231446h245bb4a9gec111f68a74a44e4@mail.gmail.com> <1243151839.26820.642.camel@twins> <63386a3d0905250513q6ca56eeepcf7bebe46c447fb4@mail.gmail.com> <1243256510.26820.679.camel@twins> <63386a3d0906010046k3b86d70av3e290fc26c2934f5@mail.gmail.com> Content-Type: text/plain Content-Transfer-Encoding: 7bit Date: Tue, 02 Jun 2009 11:00:12 +0200 Message-Id: <1243933212.23657.5726.camel@twins> Mime-Version: 1.0 X-Mailer: Evolution 2.26.1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1882 Lines: 47 On Mon, 2009-06-01 at 09:46 +0200, Linus Walleij wrote: > 2009/5/25 Peter Zijlstra : > > On Mon, 2009-05-25 at 14:13 +0200, Linus Walleij wrote: > >> 2009/5/24 Peter Zijlstra : > >> > >> > On Sat, 2009-05-23 at 23:46 +0200, Linus Walleij wrote: > >> > > >> >> This overrides the global sched_clock() symbol in the Linux > >> >> scheduler with a local implementation which takes advantage of > >> >> the timesource in U300 giving a scheduling resolution of 1us. The > >> >> solution is the same as found in the OMAP2 core code. > >> > > >> > We assume sched_clock() to return time in ns (e-9) resolution. > >> > >> Yep okay and in this case: > >> > >> >> + ret = (unsigned long long) u300_get_cycles(); > >> >> + ret = (ret * clocksource_u300_1mhz.mult_orig) >> > >> >> + clocksource_u300_1mhz.shift; > >> >> + return ret; > >> > >> (mult_orig >> shift) == 1000 > > > > Ah, ok -- missed that little detail ;-) > > > >> So for each cycle in cyclecount register we return 1000 * cycles > >> i.e 1000ns. > >> > >> If it looks nicer we can of course simply: > >> return (unsigned long long) u300_get_cycles * 1000; > >> > >> But the question here is whether this resolution is enough for > >> sched_clock() or if it is irrelevant to override sched_clock() > >> if it cannot schedule with better precision than 1000 ns. > > > > No anything better than jiffies is good, 1us certainly is worth the > > trouble. > > Can I interpret this as Acked-by: Peter Zijlstra ? I think its best if we continue with the patch Paul Mundt has been proposing. -- 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/