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=-3.8 required=3.0 tests=BAYES_00, 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 CA273C41604 for ; Tue, 6 Oct 2020 16:19:49 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 889A8206F7 for ; Tue, 6 Oct 2020 16:19:49 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726143AbgJFQTs (ORCPT ); Tue, 6 Oct 2020 12:19:48 -0400 Received: from foss.arm.com ([217.140.110.172]:51516 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725902AbgJFQTs (ORCPT ); Tue, 6 Oct 2020 12:19:48 -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 6BF6611B3; Tue, 6 Oct 2020 09:19:47 -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 6C6A33F66B; Tue, 6 Oct 2020 09:19:45 -0700 (PDT) References: <20201005145717.346020688@infradead.org> <20201005150922.458081448@infradead.org> <20201006134850.GV2628@hirez.programming.kicks-ass.net> User-agent: mu4e 0.9.17; emacs 26.3 From: Valentin Schneider To: Peter Zijlstra Cc: tglx@linutronix.de, mingo@kernel.org, linux-kernel@vger.kernel.org, bigeasy@linutronix.de, qais.yousef@arm.com, swood@redhat.com, juri.lelli@redhat.com, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de, bristot@redhat.com, vincent.donnefort@arm.com, tj@kernel.org Subject: Re: [PATCH -v2 15/17] sched: Fix migrate_disable() vs rt/dl balancing In-reply-to: <20201006134850.GV2628@hirez.programming.kicks-ass.net> Date: Tue, 06 Oct 2020 17:19:43 +0100 Message-ID: MIME-Version: 1.0 Content-Type: text/plain Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 06/10/20 14:48, Peter Zijlstra wrote: > On Tue, Oct 06, 2020 at 12:20:43PM +0100, Valentin Schneider wrote: >> >> An alternative I could see would be to prevent those piles from forming >> altogether, say by issuing a similar push_cpu_stop() on migrate_disable() >> if the next pushable task is already migrate_disable(); but that's a >> proactive approach whereas yours is reactive, so I'm pretty sure that's >> bound to perform worse. > > I think it is always possible to form pileups. Just start enough tasks > such that newer, higher priority, tasks have to preempt existing tasks. > > Also, we might not be able to place the task elsewhere, suppose we have > all our M CPUs filled with an RT task, then when the lowest priority > task has migrate_disable(), wake the highest priority task. > > Per the SMP invariant, this new highest priority task must preempt the > lowest priority task currently running, otherwise we would not be > running the M highest prio tasks. > Right, and it goes the other way around for the migrate_disable() task: if it becomes one of the M highest prio tasks, then it *must* run, and push/pulling its CPU's current away is the only way to do so... > That's not to say it might not still be beneficial from trying to avoid > them, but we must assume a pilup will occur, therefore my focus was on > dealing with them as best we can first. "Funny" all that... Thanks!