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 858A2C5AD4C for ; Thu, 23 Nov 2023 15:33:23 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1346093AbjKWPdP (ORCPT ); Thu, 23 Nov 2023 10:33:15 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:35096 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1346038AbjKWPdO (ORCPT ); Thu, 23 Nov 2023 10:33:14 -0500 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id 169DED41; Thu, 23 Nov 2023 07:33:20 -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 2682412FC; Thu, 23 Nov 2023 07:34:06 -0800 (PST) Received: from [10.57.3.62] (unknown [10.57.3.62]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 26E853F6C4; Thu, 23 Nov 2023 07:33:18 -0800 (PST) Message-ID: Date: Thu, 23 Nov 2023 15:33:16 +0000 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.15.1 Subject: Re: [PATCH v5 3/3] Documentation: arm64: Document the PMU event counting threshold feature Content-Language: en-US To: Suzuki K Poulose , Namhyung Kim Cc: linux-arm-kernel@lists.infradead.org, linux-perf-users@vger.kernel.org, will@kernel.org, mark.rutland@arm.com, Catalin Marinas , Jonathan Corbet , linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org References: <20231113112507.917107-1-james.clark@arm.com> <20231113112507.917107-4-james.clark@arm.com> <0058ca82-e55b-4490-90d0-871afce787c7@arm.com> From: James Clark In-Reply-To: <0058ca82-e55b-4490-90d0-871afce787c7@arm.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 21/11/2023 10:33, Suzuki K Poulose wrote: > On 20/11/2023 21:31, Namhyung Kim wrote: >> On Mon, Nov 13, 2023 at 3:26 AM James Clark wrote: >>> >>> Add documentation for the new Perf event open parameters and >>> the threshold_max capability file. >>> >>> Signed-off-by: James Clark >>> --- >>>   Documentation/arch/arm64/perf.rst | 56 +++++++++++++++++++++++++++++++ >>>   1 file changed, 56 insertions(+) >>> >>> diff --git a/Documentation/arch/arm64/perf.rst >>> b/Documentation/arch/arm64/perf.rst >>> index 1f87b57c2332..36b8111a710d 100644 >>> --- a/Documentation/arch/arm64/perf.rst >>> +++ b/Documentation/arch/arm64/perf.rst >>> @@ -164,3 +164,59 @@ and should be used to mask the upper bits as >>> needed. >>>      >>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/perf/arch/arm64/tests/user-events.c >>>   .. _tools/lib/perf/tests/test-evsel.c: >>>      >>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/lib/perf/tests/test-evsel.c >>> + >>> +Event Counting Threshold >>> +========================================== >>> + >>> +Overview >>> +-------- >>> + >>> +FEAT_PMUv3_TH (Armv8.8) permits a PMU counter to increment only on >>> +events whose count meets a specified threshold condition. For >>> example if >>> +threshold_compare is set to 2 ('Greater than or equal'), and the >>> +threshold is set to 2, then the PMU counter will now only increment by >>> +when an event would have previously incremented the PMU counter by 2 or >>> +more on a single processor cycle. >>> + >>> +To increment by 1 after passing the threshold condition instead of the >>> +number of events on that cycle, add the 'threshold_count' option to the >>> +commandline. >>> + >>> +How-to >>> +------ >>> + >>> +The threshold, threshold_compare and threshold_count values can be >>> +provided per event: >>> + >>> +.. code-block:: sh >>> + >>> +  perf stat -e stall_slot/threshold=2,threshold_compare=2/ \ >>> +            -e >>> dtlb_walk/threshold=10,threshold_compare=3,threshold_count/ >> >> Can you please explain this a bit more? >> >> I guess the first event counts stall_slot PMU if the event if it's >> greater than or equal to 2.  And as threshold_count is not set, >> it'd count the stall_slot as is.  E.g. it counts 3 when it sees 3. >> >> OTOH, dtlb_walk will count 1 if it sees an event less than 10. >> Is my understanding correct? > > That is correct. The behavior is described in the paragraph above. > But I agree that it would be really helpful if we explained with the > example above. > Yeah I can add a description of how the example behaves. >> >>> + >>> +And the following comparison values are supported: >>> + >>> +.. code-block:: >>> + >>> +  0: Not-equal >>> +  1: Equals >>> +  2: Greater-than-or-equal >>> +  3: Less-than >> >> So the above values are for threashold_compare, right? >> It'd be nice if it's more explicit. Yep I agree, I can label this with threshold_compare. >> >> Similarly, it'd be helpful to have a description for the >> threshold and threshold_count fields. > > Agreed. > > Suzuki > Yeah I'll add explicit descriptions for each field. Thanks for the review. > > >> >> Thanks, >> Namhyung >> >>> + >>> +The maximum supported threshold value can be read from the caps of each >>> +PMU, for example: >>> + >>> +.. code-block:: sh >>> + >>> +  cat /sys/bus/event_source/devices/armv8_pmuv3/caps/threshold_max >>> + >>> +  0x000000ff >>> + >>> +If a value higher than this is given, then it will be silently clamped >>> +to the maximum. The highest possible maximum is 4095, as the config >>> +field for threshold is limited to 12 bits, and the Perf tool will >>> refuse >>> +to parse higher values. >>> + >>> +If the PMU doesn't support FEAT_PMUv3_TH, then threshold_max will read >>> +0, and both threshold and threshold_compare will be silently ignored. >>> +threshold_max will also read as 0 on aarch32 guests, even if the host >>> +is running on hardware with the feature. >>> -- >>> 2.34.1 >>> >>> > >