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 E8ED3C00140 for ; Wed, 10 Aug 2022 12:29:17 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232265AbiHJM3Q (ORCPT ); Wed, 10 Aug 2022 08:29:16 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:54594 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232241AbiHJM3N (ORCPT ); Wed, 10 Aug 2022 08:29:13 -0400 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id A57987437F; Wed, 10 Aug 2022 05:29:12 -0700 (PDT) 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 08CDE1FB; Wed, 10 Aug 2022 05:29:13 -0700 (PDT) Received: from [10.57.13.63] (unknown [10.57.13.63]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 3EFD23F5A1; Wed, 10 Aug 2022 05:29:10 -0700 (PDT) Message-ID: <3a5e7abd-9361-11ba-978d-8e8bae00ea31@arm.com> Date: Wed, 10 Aug 2022 13:29:08 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.9.1 Subject: Re: [PATCH v2 1/1] ACPI: CPPC: Disable FIE if registers in PCC regions Content-Language: en-US To: Jeremy Linton Cc: rafael@kernel.org, lenb@kernel.org, viresh.kumar@linaro.org, robert.moore@intel.com, devel@acpica.org, linux-acpi@vger.kernel.org, linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org, vschneid@redhat.com, Ionela Voinescu , Dietmar Eggemann References: <20220728221043.4161903-1-jeremy.linton@arm.com> <20220728221043.4161903-2-jeremy.linton@arm.com> From: Lukasz Luba In-Reply-To: <20220728221043.4161903-2-jeremy.linton@arm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Jeremy, +CC Valentin since he might be interested in this finding +CC Ionela, Dietmar I have a few comments for this patch. On 7/28/22 23:10, Jeremy Linton wrote: > PCC regions utilize a mailbox to set/retrieve register values used by > the CPPC code. This is fine as long as the operations are > infrequent. With the FIE code enabled though the overhead can range > from 2-11% of system CPU overhead (ex: as measured by top) on Arm > based machines. > > So, before enabling FIE assure none of the registers used by > cppc_get_perf_ctrs() are in the PCC region. Furthermore lets also > enable a module parameter which can also disable it at boot or module > reload. > > Signed-off-by: Jeremy Linton > --- > drivers/acpi/cppc_acpi.c | 41 ++++++++++++++++++++++++++++++++++ > drivers/cpufreq/cppc_cpufreq.c | 19 ++++++++++++---- > include/acpi/cppc_acpi.h | 5 +++++ > 3 files changed, 61 insertions(+), 4 deletions(-) 1. You assume that all platforms would have this big overhead when they have the PCC regions for this purpose. Do we know which version of HW mailbox have been implemented and used that have this 2-11% overhead in a platform? Do also more recent MHU have such issues, so we could block them by default (like in your code)? 2. I would prefer to simply change the default Kconfig value to 'n' for the ACPI_CPPC_CPUFREQ_FIE, instead of creating a runtime check code which disables it. We have probably introduce this overhead for older platforms with this commit: commit 4c38f2df71c8e33c0b64865992d693f5022eeaad Author: Viresh Kumar Date: Tue Jun 23 15:49:40 2020 +0530 cpufreq: CPPC: Add support for frequency invariance If the test server with this config enabled performs well in the stress-tests, then on production server the config may be set to 'y' (or 'm' and loaded). I would vote to not add extra code, which then after a while might be decided to bw extended because actually some HW is actually capable (so we could check in runtime and enable it). IMO this create an additional complexity in our diverse configuration/tunnable space in our code. When we don't compile-in this, we should fallback to old-style FIE, which has been used on these old platforms. BTW (I have to leave it here) the first-class solution for those servers is to implement AMU counters, so the overhead to retrieve this info is really low. Regards, Lukasz