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 98810C4332D for ; Thu, 19 Mar 2020 12:23:24 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 6C40F20663 for ; Thu, 19 Mar 2020 12:23:24 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727065AbgCSMXX (ORCPT ); Thu, 19 Mar 2020 08:23:23 -0400 Received: from foss.arm.com ([217.140.110.172]:34410 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726936AbgCSMXX (ORCPT ); Thu, 19 Mar 2020 08:23:23 -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 CDBA731B; Thu, 19 Mar 2020 05:23:22 -0700 (PDT) Received: from e113632-lin (unknown [10.1.194.46]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 0ECD93F305; Thu, 19 Mar 2020 05:23:21 -0700 (PDT) References: <20200311181601.18314-1-valentin.schneider@arm.com> <20200311181601.18314-10-valentin.schneider@arm.com> <53763a32-0ce5-e267-9d5d-99e65c921d08@arm.com> User-agent: mu4e 0.9.17; emacs 26.3 From: Valentin Schneider To: Dietmar Eggemann Cc: linux-kernel@vger.kernel.org, mingo@kernel.org, peterz@infradead.org, vincent.guittot@linaro.org Subject: Re: [PATCH v2 9/9] sched/topology: Define and use shortcut pointers for wakeup sd_flag scan In-reply-to: <53763a32-0ce5-e267-9d5d-99e65c921d08@arm.com> Date: Thu, 19 Mar 2020 12:22:34 +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 Thu, Mar 19 2020, Dietmar Eggemann wrote: > On 11.03.20 19:16, Valentin Schneider wrote: >> @@ -6621,7 +6610,19 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags) >> >> rcu_read_lock(); >> >> - sd = highest_flag_domain(cpu, sd_flag); >> + switch (wake_flags & (WF_TTWU | WF_FORK | WF_EXEC)) { >> + case WF_TTWU: >> + sd_flag = SD_BALANCE_WAKE; >> + sd = rcu_dereference(per_cpu(sd_balance_wake, cpu)); > > IMHO, since we hard-code 0*SD_BALANCE_WAKE in sd_init(), sd would always > be NULL, so !want_affine (i.e. wake_wide()) would still go sis(). > > SD_BALANCE_WAKE is no a topology related sd_flag so it can't be set from > outside. Since the sd->flags sysctl is now read-only, wouldn't this case > be redundant? > On a purely mainline kernel, yes, 'sd' will always be NULL for wakeups. I'm playing a bit more conservative here with SD_BALANCE_WAKE, as I could see some folks using that flag in some ~franken~ downstream kernels (I am not aware of any, however). Also, to be fair we've only very recently gotten rid of the last SD_BALANCE_WAKE user (asymmetric CPU capacity topologies), so I felt like keeping it around for a bit before entirely killing it would be a sane thing to do. > [...]