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 D75E6C43381 for ; Thu, 7 Mar 2019 15:20:00 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id AFFE420840 for ; Thu, 7 Mar 2019 15:20:00 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726496AbfCGPT7 (ORCPT ); Thu, 7 Mar 2019 10:19:59 -0500 Received: from foss.arm.com ([217.140.101.70]:47142 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726161AbfCGPT7 (ORCPT ); Thu, 7 Mar 2019 10:19:59 -0500 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.72.51.249]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 88E68EBD; Thu, 7 Mar 2019 07:19:58 -0800 (PST) Received: from e107155-lin (e107155-lin.cambridge.arm.com [10.1.196.42]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id BD5813F706; Thu, 7 Mar 2019 07:19:56 -0800 (PST) Date: Thu, 7 Mar 2019 15:19:54 +0000 From: Sudeep Holla To: Lingutla Chandrasekhar Cc: quentin.perret@arm.com, dietmar.eggemann@arm.com, gregkh@linuxfoundation.org, will.deacon@arm.com, catalin.marinas@arm.com, morten.rasmussen@arm.com, linux-arm-kernel@lists.infradead.org, jeremy.linton@arm.com, linux-kernel@vger.kernel.org Subject: Re: [PATCH v1] arch_topology: Make cpu_capacity sysfs node as ready-only Message-ID: <20190307151954.GB5778@e107155-lin> References: <20190306152254.GB19434@e105550-lin.cambridge.arm.com> <1551886073-16217-1-git-send-email-clingutla@codeaurora.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1551886073-16217-1-git-send-email-clingutla@codeaurora.org> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Mar 06, 2019 at 08:57:53PM +0530, Lingutla Chandrasekhar wrote: > If user updates any cpu's cpu_capacity, then the new value is going to > be applied to all its online sibling cpus. But this need not to be correct > always, as sibling cpus (in ARM, same micro architecture cpus) would have > different cpu_capacity with different performance characteristics. > So updating the user supplied cpu_capacity to all cpu siblings > is not correct. > > And another problem is, current code assumes that 'all cpus in a cluster > or with same package_id (core_siblings), would have same cpu_capacity'. > But with commit '5bdd2b3f0f8 ("arm64: topology: add support to remove > cpu topology sibling masks")', when a cpu hotplugged out, the cpu > information gets cleared in its sibling cpus. So user supplied > cpu_capacity would be applied to only online sibling cpus at the time. > After that, if any cpu hot plugged in, it would have different cpu_capacity > than its siblings, which breaks the above assumption. > > So instead of mucking around the core sibling mask for user supplied > value, use device-tree to set cpu capacity. And make the cpu_capacity > node as read-only to know the assymetry between cpus in the system. > Acked-by: Sudeep Holla IIRC this was added for 2 possibilities though I don't completely agree no one had any objections(including me though I wonder how/why I missed to notice it now, anyways it's too late) 1. For systems that don't provide this information via device-tree/any firmware though that's the highly recommended way. With more complex topologies in horizon, I can't think of fetching/deducing this information *correctly* in any other sane way. 2. For some sort of tuning(avoid rebuild and reboot), but that's questionable as this is not a software characteristic. It's more like deriving hardware characteristics using software experiments. So, for me, we can compare this with some hardware latencies we have like CPU idle entry/exit latencies. They are tuned but not in production kernels. So if there's a case for adding this back as write capable sysfs, I would prefer that in debugfs and this sysfs is read-only ABI. Hope that helps. -- Regards, Sudeep