From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mel Gorman Date: Wed, 21 Oct 2020 15:08:00 +0000 Subject: Re: [PATCH] sched/fair: check for idle core Message-Id: <20201021150800.GG32041@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> <20201021131827.GF32041@suse.de> In-Reply-To: MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit 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 On Wed, Oct 21, 2020 at 03:24:48PM +0200, Julia Lawall wrote: > > 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. > > But it is equal to the original behavior in the idle prev case if you go > back to the runnable load average days... > It is similar but it misses the sync treatment and sd->imbalance_pct part of wake_affine_weight which has unpredictable consequences. The data available is only on the fully utilised case. > The problem seems impossible to solve, because there is no way to know by > looking only at prev and this whether the thread would prefer to stay > where it was or go to the waker. > Yes, this is definitely true. Looking at prev_cpu and this_cpu is a crude approximation and the path is heavily limited in terms of how clever it can be. -- Mel Gorman SUSE Labs