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=-5.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 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 7E893C47095 for ; Wed, 9 Jun 2021 07:26:04 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 58C9461354 for ; Wed, 9 Jun 2021 07:26:04 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S236853AbhFIH15 (ORCPT ); Wed, 9 Jun 2021 03:27:57 -0400 Received: from mail2-relais-roc.national.inria.fr ([192.134.164.83]:12371 "EHLO mail2-relais-roc.national.inria.fr" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S234029AbhFIH1z (ORCPT ); Wed, 9 Jun 2021 03:27:55 -0400 IronPort-HdrOrdr: =?us-ascii?q?A9a23=3Ak4Cj560mSeVZ5jLgBDZTrwqjBKckLtp133Aq?= =?us-ascii?q?2lEZdPUzSKClfqGV88jzuiWVtN98YgBFpTniAse9qA3nhP1ICOAqVN/INmTbUQ?= =?us-ascii?q?2TXeZfBODZsljd8kPFh4xgPN9bEpRDNA=3D=3D?= X-IronPort-AV: E=Sophos;i="5.83,260,1616454000"; d="scan'208";a="512502862" Received: from 173.121.68.85.rev.sfr.net (HELO hadrien) ([85.68.121.173]) by mail2-relais-roc.national.inria.fr with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 09 Jun 2021 09:26:00 +0200 Date: Wed, 9 Jun 2021 09:26:00 +0200 (CEST) From: Julia Lawall X-X-Sender: jll@hadrien To: Peter Zijlstra cc: Ingo Molnar , Vincent Guittot , Dietmar Eggemann , Mel Gorman , linux-kernel@vger.kernel.org Subject: Re: find_new_ilb In-Reply-To: Message-ID: References: User-Agent: Alpine 2.22 (DEB 394 2020-01-19) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 9 Jun 2021, Peter Zijlstra wrote: > On Tue, Jun 08, 2021 at 09:51:30PM +0200, Julia Lawall wrote: > > Starting from the following commit: > > > > commit 45da7a2b0af8fa29dff2e6ba8926322068350fce > > Author: Peter Zijlstra > > Date: Tue Aug 18 10:48:17 2020 +0200 > > > > sched/fair: Exclude the current CPU from find_new_ilb() > > > > up through Linux 5.12, I observed that often when most of the machine was > > idle, there could be many (thousands) of sched_wake_idle_without_ipi > > events, typically between cores 0 and 1. I don't see this any more in > > Linux v5.13-rc1. I looked through the patches to fair.c and core.c > > subsequent to v5.12, and I didn't see anything that explicitly addresses > > this issue. Before I plunge into another set of rounds of bisecting, I > > wonder if anyone knows whether and how this problem was resolved? > > Hurmph.. that patch was preparation for a later change that never seems > to have happened. If it is causing trouble for you, I think you can > savely revert it. > > At the time I thought it was very strange that new_idle would select > itself as idle-balancer, doubly so, because the only way to get there > would be with NEED_RESCHED already set, so the IPI wouldn't in fact do > anything. > > Looking again, the difference is ofcourse that previously we'd select > self and NO-OP, but now we'll potentially select another CPU and > actually do something. > > This is arguably an improvement, because we did want to do something. > > I can't quite remember what would've change here since, Vincent, can > you remember? > > Anyway, is this actually causing you trouble, or are you just going on > the increased number of events? Thanks for the feedback. The scenarios I saw wouldn't have caused trouble for anyone, because the machine was highly idle (maybe a couple of cores in use). And the problem seems to be gone as well in v5.13. julia