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 0747DC00140 for ; Wed, 10 Aug 2022 15:32:13 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231204AbiHJPcK (ORCPT ); Wed, 10 Aug 2022 11:32:10 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:54368 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229924AbiHJPcH (ORCPT ); Wed, 10 Aug 2022 11:32:07 -0400 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id EB455658C; Wed, 10 Aug 2022 08:32:04 -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 6E00A1424; Wed, 10 Aug 2022 08:32:05 -0700 (PDT) Received: from [10.57.42.77] (unknown [10.57.42.77]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 73B813F67D; Wed, 10 Aug 2022 08:32:02 -0700 (PDT) Message-ID: <2d0fc5d7-d484-f64c-fe50-18d18ad95fa2@arm.com> Date: Wed, 10 Aug 2022 17:32:04 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.11.0 Subject: Re: [PATCH v2 1/1] ACPI: CPPC: Disable FIE if registers in PCC regions Content-Language: en-US To: Lukasz Luba , 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> <3a5e7abd-9361-11ba-978d-8e8bae00ea31@arm.com> <4da7cd19-4b98-9360-922f-d625c4ec55e0@arm.com> From: Pierre Gondois In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 8/10/22 16:37, Lukasz Luba wrote: > > > On 8/10/22 15:30, Jeremy Linton wrote: >> Hi, >> >> On 8/10/22 07:29, Lukasz Luba wrote: >>> 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)? >> >> I posted that other email before being awake and conflated MHU with AMU >> (which could potentially expose the values directly). But the CPPC code >> isn't aware of whether a MHU or some other mailbox is in use. Either >> way, its hard to imagine a general mailbox with a doorbell/wait for >> completion handshake will ever be fast enough to consider running at the >> granularity this code is running at. If there were a case like that, the >> kernel would have to benchmark it at runtime to differentiate it from >> something that is talking over a slow link to a slowly responding mgmt >> processor. > > Exactly, I'm afraid the same, that we would never get such fast > mailbox-based platform. Newer platforms would just use AMU, so > completely different code and no one would even bother to test if > their HW mailbox is fast-enough for this FIE purpose ;) To add some platform information, the following platforms are using CPPC through PCC channels (so mailboxes): - Cavium ThunderX2 - Ampere eMAG - Ampere Altra Fwiw, I can confirm the cppc_fie kthread can represent a significant load, with a utilization between 2% and 30%. Regards, Pierre