From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932237AbcFBGiY (ORCPT ); Thu, 2 Jun 2016 02:38:24 -0400 Received: from mga02.intel.com ([134.134.136.20]:16577 "EHLO mga02.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932089AbcFBGiW (ORCPT ); Thu, 2 Jun 2016 02:38:22 -0400 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.26,404,1459839600"; d="scan'208";a="819986493" Date: Thu, 2 Jun 2016 06:41:06 +0800 From: Yuyang Du To: Mike Galbraith Cc: Peter Zijlstra , mingo@kernel.org, linux-kernel@vger.kernel.org, bsegall@google.com, pjt@google.com, morten.rasmussen@arm.com, vincent.guittot@linaro.org, dietmar.eggemann@arm.com Subject: Re: [RFC PATCH 1/2] sched: Clean up SD_BALANCE_WAKE flags in sched domain build-up Message-ID: <20160601224106.GB18670@intel.com> References: <1464657098-24880-1-git-send-email-yuyang.du@intel.com> <1464657098-24880-2-git-send-email-yuyang.du@intel.com> <20160531092146.GT3192@twins.programming.kicks-ass.net> <20160531013132.GQ18670@intel.com> <1464757633.4023.39.camel@gmail.com> <20160601000105.GU18670@intel.com> <1464773799.4023.72.camel@gmail.com> <20160601200325.GA18670@intel.com> <1464846623.3766.49.camel@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1464846623.3766.49.camel@gmail.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jun 02, 2016 at 07:50:23AM +0200, Mike Galbraith wrote: > > > Nope, those two have different meanings. We pass SD_BALANCE_WAKE to > > > identify a ttwu() wakeup, just as we pass SD_BALANCE_FORK to say we're > > > waking a child. SD_WAKE_AFFINE means exactly what it says, but is only > > > applicable to ttwu() wakeups. > > > > I don't disagree, but want to add that, SD_WAKE_AFFINE has no meaning that is so > > special and so important for anyone to use the flag to tune anything. If you want > > to do any SD_BALANCE_*, waker CPU is a valid candidate if allowed, that is it. > > That flag lets the user specifically tell us that he doesn't want us to > bounce his tasks around the box, cache misses be damned. The user may > _know_ that say cross node migrations hurt his load more than help, and > not want us to do that, thus expresses himself by turning the flag off > at whatever level. People do that. You can force them to take other > measures, but why do that? Agreed, and with this patch, just disable SD_BALANCE_WAKE. > > IIUC your XXX mark and your comment "Prefer wake_affine over balance flags", you > > said the same thing: SD_WAKE_AFFINE should be part of SD_BALANCE_WAKE, and should > > be part of all SD_BALANCE_* flags, > > Peter wrote that, but I don't read it the way you do. I read as if the > user wants the benefits of affine wakeups, he surely doesn't want us to > send the wakee off to god know where on every wakeup _instead_ of > waking affine, he wants to balance iff he can't have an affine wakeup. That is another matter within SD_BALANCE_WAKE we may further define: how much effort to scan or how frequent bouncing etc the user wants. This is now defined by SD_WAKE_AFFINE flag, which I certainly don't think is good. > > > If wake_wide() says we do not want an affine wakeup, but you apply > > > SD_WAKE_AFFINE meaning to SD_BALANCE_WAKE and turn it on in ->flags, > > > we'll give the user a free sample of full balance cost, no? > > > > Yes, and otherwise we don't select anything? That is just bad engough whether worse > > or not. So the whole fuss I made is really that this is a right thing to start with. :) > > Nope, leaving tasks where they were is not a bad thing. Lots of stuff > likes the scheduler best when it leaves them the hell alone :) That > works out well all around, balance cycles are spent in userspace > instead, scheduler produces wins by doing nothing, perfect. > Again, agreed, and with this patch, just disable SD_BALANCE_WAKE. :)