From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752804AbcD1MAS (ORCPT ); Thu, 28 Apr 2016 08:00:18 -0400 Received: from bombadil.infradead.org ([198.137.202.9]:41798 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751411AbcD1MAQ (ORCPT ); Thu, 28 Apr 2016 08:00:16 -0400 Date: Thu, 28 Apr 2016 14:00:12 +0200 From: Peter Zijlstra To: Mike Galbraith Cc: Chris Mason , Ingo Molnar , Matt Fleming , linux-kernel@vger.kernel.org Subject: Re: [PATCH RFC] select_idle_sibling experiments Message-ID: <20160428120012.GZ3430@twins.programming.kicks-ass.net> References: <20160405180822.tjtyyc3qh4leflfj@floor.thefacebook.com> <1459927644.5612.41.camel@suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1459927644.5612.41.camel@suse.de> 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 Wed, Apr 06, 2016 at 09:27:24AM +0200, Mike Galbraith wrote: > sched: ratelimit nohz > > Entering nohz code on every micro-idle is too expensive to bear. > > Signed-off-by: Mike Galbraith > +int sched_needs_cpu(int cpu) > +{ > + if (tick_nohz_full_cpu(cpu)) > + return 0; > + > + return cpu_rq(cpu)->avg_idle < sysctl_sched_migration_cost; So the only problem I have with this patch is the choice of limit. This isn't at all tied to the migration cost. And some people are already twiddling with the migration_cost knob to affect the idle_balance() behaviour -- making it much more agressive by dialing it down. When you do that you also loose the effectiveness of this proposed usage, even though those same people would probably want this. Failing a spot of inspiration for a runtime limit on this; we might have to introduce yet another knob :/