From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753915AbeBLTO4 convert rfc822-to-8bit (ORCPT ); Mon, 12 Feb 2018 14:14:56 -0500 Received: from mout.gmx.net ([212.227.15.15]:47605 "EHLO mout.gmx.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752972AbeBLTOy (ORCPT ); Mon, 12 Feb 2018 14:14:54 -0500 Message-ID: <1518462889.7104.78.camel@gmx.de> Subject: Re: [PATCH 3/4] sched/fair: Do not migrate on wake_affine_weight if weights are equal From: Mike Galbraith To: Peter Zijlstra , Mel Gorman Cc: Matt Fleming , LKML Date: Mon, 12 Feb 2018 20:14:49 +0100 In-Reply-To: <20180212172926.GY25201@hirez.programming.kicks-ass.net> References: <20180212145857.8056-1-mgorman@techsingularity.net> <20180212145857.8056-4-mgorman@techsingularity.net> <20180212172926.GY25201@hirez.programming.kicks-ass.net> Content-Type: text/plain; charset="ISO-8859-15" X-Mailer: Evolution 3.20.5 Mime-Version: 1.0 Content-Transfer-Encoding: 8BIT X-Provags-ID: V03:K0:4XU3C0IQrIcGRD6EMQIQiHFUOz4oFN4LvvcsXyXg42fzg84sRFe sOwPQZ1TZO9QQsXLQLRqJ/4VwlR00ix1nEdBqWgZbhQcyNdA49iSVb6mcnhwWYV2AlNB/FB eBt0+WkfySccbz7vqbweRwdVjuXBGqta1x3BaRaGFmixxFRjQva0XPOKVCGeG1SJe0Ibt7n oy/AY7qf+GpJUaaOM3sMA== X-UI-Out-Filterresults: notjunk:1;V01:K0:hJwS760XKns=:wE+/bWa0rdSJyki4Yfbx1U 0noRhVFsGltDxmqsWQLxQcLK11OIwBLW0EIvubHn+XNJ2O/WE3XMHyBEV8/S6ovoXa7jTO6v1 UE35+OHnr83+YrO+u9EnKb6QWU/MLWlxz+3gb2NWNl1zPbZkMH8v/JoqbUc9frvpu/vLLyhCW jQtb3raWMXNtCtmEBMXpD7bxDGay4jTboAcVVLFktyH/QwT7TBflBfuN5YMOVK8SbQoLzDmMd EJu+kU8r5TpIZdNEmY6f6kjOLh1X7GaJc3OBcB5PN5S+dcLDPVredEba3HypnoaZ7RCyTCmeA b2qUM/tARU0N4UzM20vQaWcScxKgLNM9CUpjZCiX2GmGVHsTyOBUXXTWsfpu35Q6Fl13Ljs4Y aWGsxdOC5Amze9XOidvJfIe9YVCG8ToiuTw3NoVn7NsC3GDcew86E+GFa6zVKa6ueThkserZ+ pB5NpBSIQsjOdbxxt8iZfc/ZfJynRkPIuTQwl/ilSQkL4vK2PTTQyPB9IGcQh+9mHBJ0COZQ9 KSNcZJFA89sH98SzgpUVZ1FxudalCj9n7nKN9QUZAXLkKc0jU9C1nNg/GaZ7qp4BGLdD2/LoC 2Mda0fVwyRs11uPBVe6tlTTEvATRqF0ZHorKWLT11LtrZq0l5xcC+sm64wtbNUMcD8sv1jUef Je4JnyhdyilQYo79kZzqjAG/L2itTCi+2lS2SmTrbTkwpusdvCqP+VhNj0l/EfD+apk7N3M+b 1F9v+/aHTQRK0wf973aqvFFQv6QjJE4hue1sfRp8+EG1yOeT0PtluBI+0ydasLgHO0+oEMiXM eec4A0Sz6paTF5jEKP/hBayXMufeH1EAbfoGQaZ1LO4Bdf6pbw= Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 2018-02-12 at 18:29 +0100, Peter Zijlstra wrote: > On Mon, Feb 12, 2018 at 02:58:56PM +0000, Mel Gorman wrote: > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > > index c1091cb023c4..28c8d9c91955 100644 > > --- a/kernel/sched/fair.c > > +++ b/kernel/sched/fair.c > > @@ -5747,7 +5747,16 @@ wake_affine_weight(struct sched_domain *sd, struct task_struct *p, > > prev_eff_load *= 100 + (sd->imbalance_pct - 100) / 2; > > prev_eff_load *= capacity_of(this_cpu); > > > > - return this_eff_load <= prev_eff_load ? this_cpu : nr_cpumask_bits; > > + /* > > + * If sync, adjust the weight of prev_eff_load such that if > > + * prev_eff == this_eff that select_idle_sibling will consider > > + * stacking the wakee on top of the waker if no other CPU is > > + * idle. > > + */ > > + if (sync) > > + prev_eff_load += 1; > > So where we had <= and would consistently favour pulling the task to the > waking CPU when all else what equal, you now switch to <, such that when > things are equal we do not pull. > > That makes sense I suppose. > > Except for sync wakeups, where you say, if everything else is equal, > pull, which also makes sense, because sync says 'current' promises to go > away. > > OK. Tasks tend to not honor that promise.. a lot.  Even if the sync hint were a golden promise, it wouldn't be bullet proof: migrating a compute hog based on a single "phoned home" wakeup remains a bad idea whether schedule() is called immediately after the wakeup or not.  It's a pain, only useful informational content is "this is a communication wakeup". -Mike