From mboxrd@z Thu Jan 1 00:00:00 1970 From: Julia Lawall Date: Wed, 21 Oct 2020 13:48:59 +0000 Subject: Re: [PATCH] sched/fair: check for idle core Message-Id: MIME-Version: 1 Content-Type: multipart/mixed; boundary="8323329-484522562-1603288140=:57356" 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> <20201021131827.GF32041@suse.de> In-Reply-To: <20201021131827.GF32041@suse.de> To: Mel Gorman Cc: Julia Lawall , 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 This message is in MIME format. The first part should be readable text, while the remaining parts are likely unreadable without MIME-aware tools. --8323329-484522562-1603288140=:57356 Content-Type: text/plain; charset="iso-8859-15" Content-Transfer-Encoding: quoted-printable On Wed, 21 Oct 2020, Mel Gorman wrote: > On Wed, Oct 21, 2020 at 02:56:06PM +0200, Julia Lawall wrote: > > > > > > On Wed, 21 Oct 2020, Mel Gorman wrote: > > > > > 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. = Vincent, > > > > > did I miss something stupid? > > > > > > > > This patch fixes the problem that we don't favor anymore the prev_c= pu when it is idle since > > > > commit 11f10e5420f6ce because load is not null when cpu is idle whe= reas 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= the > > > load on the waker CPU or its available, we care about whether it has = idle > > > siblings that can be found quickly. As tempting as ripping it out is, > > > it never happened because sometimes it makes the right decision. > > > > My goal was to restore the previous behavior, when runnable load was us= ed. > > 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. > > > > Which is fair. > > > Is your point of view that the proposed change is overkill? Or is it t= hat > > the original behavior was not desirable? > > > > 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. Could it be possible to check p->recent_used_cpu? If that is prev (or on the same socket?), then prev could be a good choice. If that is on the same socket as the waker, then maybe the waker would be better. julia --8323329-484522562-1603288140=:57356--