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=-2.3 required=3.0 tests=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 A4664ECE58F for ; Tue, 15 Oct 2019 09:22:21 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 822CA2064B for ; Tue, 15 Oct 2019 09:22:21 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729759AbfJOJWU (ORCPT ); Tue, 15 Oct 2019 05:22:20 -0400 Received: from foss.arm.com ([217.140.110.172]:33332 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725819AbfJOJWT (ORCPT ); Tue, 15 Oct 2019 05:22:19 -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 1A33928; Tue, 15 Oct 2019 02:22:19 -0700 (PDT) Received: from [192.168.0.9] (unknown [172.31.20.19]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id B04AB3F718; Tue, 15 Oct 2019 02:22:17 -0700 (PDT) Subject: Re: [PATCH] sched/topology: Disable sched_asym_cpucapacity on domain destruction To: Valentin Schneider , Quentin Perret Cc: Vincent Guittot , linux-kernel , Ingo Molnar , Peter Zijlstra , Morten Rasmussen , Quentin Perret , "# v4 . 16+" References: <20191014114710.22142-1-valentin.schneider@arm.com> <20191014121648.GA53234@google.com> <20191014135256.GA85340@google.com> <2b058430-1951-3d58-ebf4-8195a28ff233@arm.com> From: Dietmar Eggemann Message-ID: Date: Tue, 15 Oct 2019 11:22:12 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0 MIME-Version: 1.0 In-Reply-To: <2b058430-1951-3d58-ebf4-8195a28ff233@arm.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 14/10/2019 18:03, Valentin Schneider wrote: > On 14/10/2019 14:52, Quentin Perret wrote: >> Right, but that's not possible by definition -- static keys aren't >> variables. The static keys for asym CPUs and for EAS are just to >> optimize the case when they're disabled, but when they _are_ enabled, >> you have no choice but do another per-rd check. >> > > Bleh, right, realized my nonsense after sending the email. > >> And to clarify what I tried to say before, it might be possible to >> 'count' the number of RDs that have SD_ASYM_CPUCAPACITY set using >> static_branch_inc()/dec(), like we do for the SMT static key. I remember >> trying to do something like that for EAS, but that was easier said than >> done ... :) >> > > Gotcha, the reason I didn't go with this is that I wanted to maintain > the relationship between the key and the flag (you either have both or none). > It feels icky to have the key set and to have a NULL sd_asym_cpucapacity > pointer. > > An alternative might be to have a separate counter for asymmetric rd's, > always disable the key on domain destruction and use that counter to figure > out if we need to restore it. If we don't care about having a NULL SD > pointer while the key is set, we could use the included counter as you're > suggesting. I still don't understand the benefit of the counter approach here. sched_smt_present counts the number of cores with SMT. So in case you have 2 SMT cores with 2 HW threads and you CPU hp out one CPU, you still have sched_smt_present, although 1 CPU doesn't have a SMT thread sibling anymore. Valentin's patch makes sure that sched_asym_cpucapacity is correctly set when the sd hierarchy is rebuild due to CPU hp. Including the unlikely scenario that an asymmetric CPU capacity system (based on DT's capacity-dmips-mhz values) turns normal SMT because of the max frequency values of the CPUs involved. Systems with a mix of asymmetric and symmetric CPU capacity rd's have to live with the fact that wake_cap and misfit handling is enabled for them. This should be the case already today. There should be no SD_ASYM_CPUCAPACITY flag on the sd's of the CPUs of the symmetric CPU capacity rd's. I.e. update_top_cache_domain() should set sd_asym_cpucapacity=NULL for those CPUs. So as a rule we could say even if a static key enables a code path, a derefenced sd still has to be checked against NULL.