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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 3FB27C4167B for ; Thu, 7 Dec 2023 09:23:44 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1378543AbjLGJXf (ORCPT ); Thu, 7 Dec 2023 04:23:35 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:59328 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1378534AbjLGJXH (ORCPT ); Thu, 7 Dec 2023 04:23:07 -0500 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id 2D2592113; Thu, 7 Dec 2023 01:22:09 -0800 (PST) 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 D895A12FC; Thu, 7 Dec 2023 01:22:54 -0800 (PST) Received: from localhost (ionvoi01-desktop.cambridge.arm.com [10.2.78.69]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 744783F6C4; Thu, 7 Dec 2023 01:22:08 -0800 (PST) Date: Thu, 7 Dec 2023 09:22:06 +0000 From: Ionela Voinescu To: Sumit Gupta Cc: Beata Michalska , linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-pm@vger.kernel.org, sudeep.holla@arm.covm, will@kernel.org, catalin.marinas@arm.com, viresh.kumar@linaro.org, rafael@kernel.org, yang@os.amperecomputing.com, linux-tegra@vger.kernel.org Subject: Re: [PATCH v2 2/2] cpufreq: Wire-up arch-flavored freq info into cpufreq_verify_current_freq Message-ID: References: <20231127160838.1403404-1-beata.michalska@arm.com> <20231127160838.1403404-3-beata.michalska@arm.com> <3e6077bb-907c-057f-0896-d0a5814a4229@nvidia.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wednesday 06 Dec 2023 at 18:58:17 (+0530), Sumit Gupta wrote: > > > On 05/12/23 16:35, Ionela Voinescu wrote: > > External email: Use caution opening links or attachments > > > > > > Hi Sumit, > > > > On Friday 01 Dec 2023 at 18:32:10 (+0530), Sumit Gupta wrote: > > > Hi Ionela, > > > > > > > > --- a/drivers/cpufreq/cpufreq.c > > > > > +++ b/drivers/cpufreq/cpufreq.c > > > > > @@ -1756,7 +1756,8 @@ static unsigned int cpufreq_verify_current_freq(struct cpufreq_policy *policy, b > > > > > { > > > > > unsigned int new_freq; > > > > > > > > > > - new_freq = cpufreq_driver->get(policy->cpu); > > > > > + new_freq = arch_freq_get_on_cpu(policy->cpu); > > > > > + new_freq = new_freq ?: cpufreq_driver->get(policy->cpu); > > > > > > > > Given that arch_freq_get_on_cpu() is an average frequency, it does not > > > > seem right to me to trigger the sync & update process of > > > > cpufreq_verify_current_freq() based on it. > > > > > > > > cpufreq_verify_current_freq() will at least modify the internal state of > > > > the policy and send PRE and POST notifications, if not do a full frequency > > > > update, based on this average frequency, which is likely different from > > > > the current frequency, even beyond the 1MHz threshold. > > > > > > > > While I believe it's okay to return this average frequency in > > > > cpuinfo_cur_freq, I don't think it should be used as an indication of > > > > an accurate current frequency, which is what > > > > cpufreq_verify_current_freq() expects. > > > > > > > > Sumit, can you give more details on the issue at [1] and why this change > > > > fixes it? > > > > > > > > [1] https://lore.kernel.org/lkml/6a5710f6-bfbb-5dfd-11cd-0cd02220cee7@nvidia.com/ > > > > > > > > Thank you, > > > > Ionela. > > > > > > > cpufreq_verify_current_freq() also updates 'policy->cur' in POST > > > notification if the frequency from hardware has more delta (out of sync). > > > > > > As the value from 'cpufreq_driver->get()' is not reliable due to [1], > > > calling the 'get' hook can update the 'policy->cur' with a wrong value when > > > governor starts in cpufreq_start_governor(). > > > And if the frequency is never changed after the governor starts during > > > boot e.g. when performance governor is set as default, then > > > 'scaling_cur_freq' always returns wrong value. > > > > > > Instead, the arch_freq_get_on_cpu() API updates 'policy->cur' with a more > > > stable freq value. > > > > > > [1] https://lore.kernel.org/lkml/20230418113459.12860-7-sumitg@nvidia.com/ > > > > Got it, many thanks! > > > > As the code is right now in v2, arch_freq_get_on_cpu() is called on > > show_scaling_cur_freq(), so the problem you describe would not show up. > > policy->cur would still be incorrect, but 'scaling_cur_freq' would > > return the value from arch_freq_get_on_cpu(). > > > > Would it be enough if arch_freq_get_on_cpu() gets also called from > > show_cpuinfo_cur_freq() instead of cpufreq_verify_current_freq()? > > > > Thanks, > > Ionela. > > > > Yes. > I am not sure if making both the nodes 'scaling_cur_freq' and > 'cpuinfo_cur_freq' same is fine? That would happen anyway if arch_freq_get_on_cpu() is called from cpufreq_verify_current_freq(). In principle, according to [1], it would be correct to use it for 'cpuinfo_cur_freq' and not 'scaling_cur_freq'. But the call from show_scaling_cur_freq() is already there before these patches, introduced a long time ago for x86. The topic was discussed at [2] and the agreement so far was that it would be best to keep the behaviour the same for both x86 and arm. I don't like going against the user-guide, but these patches don't actually go against the user-guide. The old call to arch_freq_get_on_cpu() from show_scaling_cur_freq() goes against it. But I agree that's something necessary to keep, as legacy for x86. Additionally, you also mentioned that you'd prefer to have a more accurate frequency returned for 'scaling_cur_freq'. [1] https://www.kernel.org/doc/Documentation/cpu-freq/user-guide.txt [2] https://lore.kernel.org/lkml/20230609043922.eyyqutbwlofqaddz@vireshk-i7/ Thanks, Ionela. > > Best Regards, > Sumit Gupta