From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 8AF8CC2BB1D for ; Fri, 13 Mar 2020 15:47:29 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 6AA1020746 for ; Fri, 13 Mar 2020 15:47:29 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727460AbgCMPr2 (ORCPT ); Fri, 13 Mar 2020 11:47:28 -0400 Received: from foss.arm.com ([217.140.110.172]:59352 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726974AbgCMPr1 (ORCPT ); Fri, 13 Mar 2020 11:47:27 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id A6A8931B; Fri, 13 Mar 2020 08:47:26 -0700 (PDT) Received: from e113632-lin (e113632-lin.cambridge.arm.com [10.1.194.46]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 710933F6CF; Fri, 13 Mar 2020 08:47:25 -0700 (PDT) References: <20200312165429.990-1-vincent.guittot@linaro.org> User-agent: mu4e 0.9.17; emacs 26.3 From: Valentin Schneider To: Vincent Guittot Cc: Ingo Molnar , Peter Zijlstra , Juri Lelli , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , linux-kernel Subject: Re: [PATCH] sched/fair: improve spreading of utilization In-reply-to: Date: Fri, 13 Mar 2020 15:47:17 +0000 Message-ID: MIME-Version: 1.0 Content-Type: text/plain Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Mar 13 2020, Vincent Guittot wrote: >> > And with more coffee that's another Doh, ASYM_PACKING would end up as >> > migrate_task. So this only affects the reduced capacity migration, which >> >> yes ASYM_PACKING uses migrate_task and the case of reduced capacity >> would use it too and would not be impacted by this patch. I say >> "would" because the original rework of load balance got rid of this >> case. I'm going to prepare a separate fix for this > > After more thought, I think that we are safe for reduced capacity too > because this is handled in the migrate_load case. In my previous > reply, I was thinking of the case where rq is not overloaded but cpu > has reduced capacity which is not handled. But in such case, we don't > have to force the migration of the task because there is still enough > capacity otherwise rq would be overloaded and we are back to the case > already handled > Good point on the capacity reduction vs group_is_overloaded. That said, can't we also reach this with migrate_task? Say the local group is entirely idle, and the busiest group has a few non-idle CPUs but they all have at most 1 running task. AFAICT we would still go to calculate_imbalance(), and try to balance out the number of idle CPUs. If the migration_type is migrate_util, that can't happen because of this change. Since we have this progressive balancing strategy (tasks -> util -> load), it's a bit odd to have this "gap" in the middle where we get one less possibility to trigger active balance, don't you think? That is, providing I didn't say nonsense again :) It's not a super big deal, but I think it's nice if we can maintain a consistent / gradual migration policy. >> >> > might be hard to notice in benchmarks.