From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mel Gorman Date: Wed, 21 Oct 2020 13:18:27 +0000 Subject: Re: [PATCH] sched/fair: check for idle core Message-Id: <20201021131827.GF32041@suse.de> List-Id: References: <1603211879-1064-1-git-send-email-Julia.Lawall@inria.fr> <20201021112038.GC32041@suse.de> <20201021122532.GA30733@vingu-book> <20201021124700.GE32041@suse.de> In-Reply-To: MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable To: Julia Lawall Cc: Vincent Guittot , Ingo Molnar , kernel-janitors@vger.kernel.org, Peter Zijlstra , Juri Lelli , Dietmar Eggemann , Steven Rostedt , Ben Segall , Daniel Bristot de Oliveira , linux-kernel@vger.kernel.org, Valentin Schneider , Gilles.Muller@inria.fr On Wed, Oct 21, 2020 at 02:56:06PM +0200, Julia Lawall wrote: >=20 >=20 > On Wed, 21 Oct 2020, Mel Gorman wrote: >=20 > > On Wed, Oct 21, 2020 at 02:25:32PM +0200, Vincent Guittot wrote: > > > > I see Vincent already agreed with the patch so I could be wrong. V= incent, > > > > did I miss something stupid? > > > > > > This patch fixes the problem that we don't favor anymore the prev_cpu= when it is idle since > > > commit 11f10e5420f6ce because load is not null when cpu is idle where= as runnable_load was > > > And this is important because this will=A0then decide in which LLC we= will looks for a cpu > > > > > > > Ok, that is understandable but I'm still concerned that the fix simply > > trades one problem for another by leaving related tasks remote to each > > other and increasing cache misses and remote data accesses. > > > > wake_affine_weight is a giant pain because really we don't care about t= he > > load on the waker CPU or its available, we care about whether it has id= le > > siblings that can be found quickly. As tempting as ripping it out is, > > it never happened because sometimes it makes the right decision. >=20 > My goal was to restore the previous behavior, when runnable load was used. > The patch removing the use of runnable load (11f10e5420f6) presented it > basically as that load balancing was using it, so wakeup should use it > too, and any way it didn't matter because idle CPUS were checked for > anyway. >=20 Which is fair. > Is your point of view that the proposed change is overkill? Or is it that > the original behavior was not desirable? >=20 I worry it's overkill because prev is always used if it is idle even if it is on a node remote to the waker. It cuts off the option of a wakee moving to a CPU local to the waker which is not equivalent to the original behaviour. --=20 Mel Gorman SUSE Labs