From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752286AbcD1NRt (ORCPT ); Thu, 28 Apr 2016 09:17:49 -0400 Received: from mx2.suse.de ([195.135.220.15]:60799 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750867AbcD1NRs (ORCPT ); Thu, 28 Apr 2016 09:17:48 -0400 Message-ID: <1461849466.3841.17.camel@suse.de> Subject: Re: [PATCH RFC] select_idle_sibling experiments From: Mike Galbraith To: Peter Zijlstra Cc: Chris Mason , Ingo Molnar , Matt Fleming , linux-kernel@vger.kernel.org Date: Thu, 28 Apr 2016 15:17:46 +0200 In-Reply-To: <20160428120012.GZ3430@twins.programming.kicks-ass.net> References: <20160405180822.tjtyyc3qh4leflfj@floor.thefacebook.com> <1459927644.5612.41.camel@suse.de> <20160428120012.GZ3430@twins.programming.kicks-ass.net> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.16.5 Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 2016-04-28 at 14:00 +0200, Peter Zijlstra wrote: > 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. Yup. > 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 :/ I'll roll one with a yet another of it's very own. -Mike