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=-2.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT autolearn=ham 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 02BADC43387 for ; Wed, 9 Jan 2019 10:07:57 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id C17F221738 for ; Wed, 9 Jan 2019 10:07:56 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730123AbfAIKH4 (ORCPT ); Wed, 9 Jan 2019 05:07:56 -0500 Received: from outbound-smtp04.blacknight.com ([81.17.249.35]:57234 "EHLO outbound-smtp04.blacknight.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729896AbfAIKHz (ORCPT ); Wed, 9 Jan 2019 05:07:55 -0500 Received: from mail.blacknight.com (pemlinmail05.blacknight.ie [81.17.254.26]) by outbound-smtp04.blacknight.com (Postfix) with ESMTPS id 8681E98B69 for ; Wed, 9 Jan 2019 10:07:53 +0000 (UTC) Received: (qmail 12814 invoked from network); 9 Jan 2019 10:07:53 -0000 Received: from unknown (HELO techsingularity.net) (mgorman@techsingularity.net@[37.228.229.96]) by 81.17.254.9 with ESMTPSA (AES256-SHA encrypted, authenticated); 9 Jan 2019 10:07:53 -0000 Date: Wed, 9 Jan 2019 10:07:51 +0000 From: Mel Gorman To: Mike Galbraith Cc: Andrea Arcangeli , Peter Zijlstra , linux-kernel@vger.kernel.org Subject: Re: [PATCH 0/1] RFC: sched/fair: skip select_idle_sibling() in presence of sync wakeups Message-ID: <20190109100751.GQ31517@techsingularity.net> References: <20190109034941.28759-1-aarcange@redhat.com> <1547007588.17100.17.camel@gmx.de> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: <1547007588.17100.17.camel@gmx.de> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jan 09, 2019 at 05:19:48AM +0100, Mike Galbraith wrote: > > To test I used this trivial program. > > Which highlights the problem. That proggy really is synchronous, but > the sync hint is applied to many MANY real world cases where this is > not the case at all. Sure, you can make things like pipe_test and > nearly nonexistent payload TCP_RR numbers look gorgeous, but that > demolishes concurrency for real applications. > I agree with Mike here. Many previous attempts to strictly obey the strict hint has led to regressions elsewhere -- specifically a task waking 2+ wakees that temporarily stack on one CPU when nearby CPUs sharing LLC remain idle. It's why the select idle sibling logic tried to take into account a recently used CPU to wake such tasks if the recent CPU was still idle. -- Mel Gorman SUSE Labs