Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932545AbaGSSbh (ORCPT ); Sat, 19 Jul 2014 14:31:37 -0400 Received: from casper.infradead.org ([85.118.1.10]:48955 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932161AbaGSSbg (ORCPT ); Sat, 19 Jul 2014 14:31:36 -0400 Date: Sat, 19 Jul 2014 20:31:32 +0200 From: Peter Zijlstra To: Nicolas Pitre Cc: Frederic Weisbecker , LKML , Ingo Molnar , "Paul E. McKenney" , Steven Rostedt , Thomas Gleixner , Viresh Kumar Subject: Re: [PATCH 07/10] nohz: Enforce timekeeping on CPU 0 Message-ID: <20140719183132.GK3935@laptop> References: <1405730661-9355-1-git-send-email-fweisbec@gmail.com> <1405730661-9355-8-git-send-email-fweisbec@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, Jul 19, 2014 at 01:31:25PM -0400, Nicolas Pitre wrote: > On Sat, 19 Jul 2014, Frederic Weisbecker wrote: > > > The timekeeper gets initialized to the value of the CPU where the > > first clockevent device is setup. This works well because the timekeeper > > can be any online CPU in most configs. > > > > Full dynticks has its own requirement though and needs the timekeeper > > to always be 0. And this requirement seem to accomodate pretty well with > > the above described boot timekeeper setting because the first clockevent > > device happens to be initialized, most of the time, on the boot CPU > > (which should be CPU 0). > > This might have been discussed before... but this isn't ARM big.LITTLE > friendly at all. > > Could we accommodate for any arbitrary CPU instead of making CPU 0 > "special" other than its role as the boot CPU please? It doesn't have > to be completely dynamic, but CPU 0 might be a really bad choice for > ongoing periodic duties in some configurations. For example, we might > highly prefer to do this on CPU 4 for power efficiency reasons once it > is online and keep CPU 0 in a deep C-state as much as possible. This is because CPU0 can be a big core, right? IIRC this is done because a big core as boot cpu, boots faster and some people think boot time is relevant. -- 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/