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 bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 03880C43458 for ; Thu, 9 Jul 2026 06:08:12 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:List-Subscribe:List-Help :List-Post:List-Archive:List-Unsubscribe:List-Id:Content-Transfer-Encoding: Content-Type:In-Reply-To:From:References:Cc:To:Subject:MIME-Version:Date: Message-ID:Reply-To:Content-ID:Content-Description:Resent-Date:Resent-From: Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=uc9kU+ZgE5XODZsQBZ86ADvj/Ljvow0ueROlYCHvuAs=; b=b1vHuPZVRqa1gImpMS0dWESLSs BXeXWVPHS0YjdYbVhPH437jezLm4+Tc/OO7aRZqObGUzd9YlQRfM2+CrqEnU51/4PSh7TUUxD7t9g YiPQI4zsDvPkGi+/mn5JtlXmJ0Zy2KfUZfvvWOWQDR0GeZUgJDhuW+OSvJFz2yETMmgLVkB6GKCOh feH8EIg6gR1BRMzmqtdvMRiQpYzqdHFL2Q+SEHPPlGGXYGnyXgPFUFTGHCpdWpPWrq0qNT/zcd1QA 1VycVtKYCVE87yPrTv1RNqpQZ5oc9UJiMLMvl0aIHqSaQqB5nz6/Wu4NWLrlYsDLHlB4SmVvausUU 3TvKrx/w==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.99.1 #2 (Red Hat Linux)) id 1whhvZ-000000016zo-1KKM; Thu, 09 Jul 2026 06:08:05 +0000 Received: from foss.arm.com ([217.140.110.172]) by bombadil.infradead.org with esmtp (Exim 4.99.1 #2 (Red Hat Linux)) id 1whhvW-000000016yW-29tS for linux-arm-kernel@lists.infradead.org; Thu, 09 Jul 2026 06:08:04 +0000 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 96D9A24C0; Wed, 8 Jul 2026 23:07:51 -0700 (PDT) Received: from [192.168.20.93] (usa-sjc-mx-foss1.foss.arm.com [172.31.20.19]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 395FA3F7B4; Wed, 8 Jul 2026 23:07:53 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=arm.com; s=foss; t=1783577275; bh=5TwAOLITC1YXoDg2mMcYupgv1n+wMrm66cLoo63ucz8=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=Ca0NrPS7WhWjH0EbrkfJ9CcgmZLP+Ci/Ix90jXf6EcrUjae7YWOHfzmh8TLgWybZL mDy49MvgdKaIAQ+/FGWqal/giD0eUa2auN8dnkFFt6ShdzlIe/aWHzlXJ9xeZX7eHM nA2t+0KITyq8j4vxmk0rTr15Zzt6ycKUNrq4h78w= Message-ID: <29947b61-d2f3-4161-ac65-197cfe00a99d@arm.com> Date: Thu, 9 Jul 2026 01:07:43 -0500 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 0/2] CPPC: reduce FFH feedback-counter sampling skew on arm64 To: Pengjie Zhang , catalin.marinas@arm.com, will@kernel.org, rafael@kernel.org, lenb@kernel.org, saket.dumbre@intel.com, beata.michalska@arm.com, zhenglifeng1@huawei.com, sumitg@nvidia.com, zhanjie9@hisilicon.com, geert+renesas@glider.be, cuiyunhui@bytedance.com, vanshikonda@os.amperecomputing.com, ionela.voinescu@arm.com, viresh.kumar@linaro.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-acpi@vger.kernel.org, acpica-devel@lists.linux.dev, linuxarm@huawei.com Cc: prime.zeng@hisilicon.com, wanghuiqiang@huawei.com, xuwei5@huawei.com, lihuisong@huawei.com, yubowen8@huawei.com, wangzhi12@huawei.com References: <20260708082818.808041-1-zhangpengjie2@huawei.com> Content-Language: en-US From: Jeremy Linton In-Reply-To: <20260708082818.808041-1-zhangpengjie2@huawei.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.9.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20260708_230802_751756_18AA7EF2 X-CRM114-Status: GOOD ( 21.18 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org Hi, On 7/8/26 3:28 AM, Pengjie Zhang wrote: > The legacy CPPC feedback-counter path reads the delivered and reference > performance counters separately. > > On arm64 systems using AMU-backed CPPC FFH counters, each FFH read is > served through a cross-CPU counter read helper. Reading the counters > separately therefore widens the sampling window between them and can > skew the delivered/reference ratio used by cpuinfo_cur_freq. Under heavy > load, the skew is observable as transient values that may exceed the > platform maximum, as discussed in [1] and [2]. > > This series adds a small generic hook for architectures that can obtain > both FFH feedback counters in one operation, while preserving the > existing per-register read path as the fallback. > > Patch 1 adds the generic CPPC hook and uses it from cppc_get_perf_ctrs(). > Patch 2 implements the hook on arm64 by sampling both AMU counters in a > single operation on the target CPU. > > For detailed test results and data demonstrating the observable skew and > the improvements brought by this series, please refer to [3] and [4]. So, this set appears to help considerably, I have a small script which after setting a fixed CPU frequency proceeds to sample cpuinfo_cur_freq and builds the mean/stddev/stderr and worst case error percentage. Its really the latter that I've been focusing on. When run under varying workloads, local vs cross cpu, t builds what I think is a somewhat reasonable picture of the machine behaviors. Given a machine that at a baseline, has an error percentage that can exceed 100% (ex the actual frequency is 2.4Ghz, and it can report > 5Ghz), this patch appears to bring the worst case down to approximately ~20%, but usually its somewhere around 5%. This is similar to the v1 of the jitter patch I posted, but worse than than the best sample version of the jitter patch I've been promising, which does a better job selecting the initial sample. But combining them is magic, the tweaked version reduces that to less than a percent. The combination appears good enough to consistently detect small variations between the requested frequencies and the rate the delivered clock is claiming (ex request 2600 Mhz, consistently get 2630 Mhz +- 2Mhz). With that: Tested-by: Jeremy Linton I have a small nitpik for review, that should only be considered if for some reason this gets respun, but otherwise. Reviewed-by: Jeremy Linton > > [1] https://lore.kernel.org/all/20231025093847.3740104-4-zengheng4@huawei.com/ > [2] https://lore.kernel.org/all/20231212072617.14756-1-lihuisong@huawei.com/ > [3] https://lore.kernel.org/all/443104e2-ba6e-454e-8469-909f35817a99@huawei.com/ > [4] https://lore.kernel.org/all/317d33d5-8279-4aa8-84b7-6ae1976636ac@huawei.com/ > > Tested-by: Sumit Gupta > Reviewed-by: Sumit Gupta > Tested-by: Vanshidhar Konda > Reviewed-by: Vanshidhar Konda > Signed-off-by: Pengjie Zhang > --- > Changes in v2: > - Simplified the CPPC generic layer fallback logic to prevent pointless single-read retries. > - Added upfront register validation in the arm64 hook to avoid unnecessary IPI overhead. > - Explicitly flipped the -EOPNOTSUPP error to -ENODEV in the arm64 hook when AMU is unsupported, cleanly bypassing redundant CPPC generic fallbacks. > - Addressed other kernel-doc and naming feedbacks from Beata. > - Added Reviewed-by and Tested-by tags from Vanshidhar and Sumit > - Link to v1: https://lore.kernel.org/all/20260410094145.4132082-1-zhangpengjie2@huawei.com/ > > Pengjie Zhang (2): > ACPI: CPPC: add paired FFH feedback-counter read hook > arm64: topology: read CPPC FFH feedback counters in one operation > > arch/arm64/kernel/topology.c | 92 ++++++++++++++++++++++++++++++++---- > drivers/acpi/cppc_acpi.c | 50 ++++++++++++++++++-- > include/acpi/cppc_acpi.h | 7 +++ > 3 files changed, 136 insertions(+), 13 deletions(-) >