netdev - Re: [PATCH] perf record/arm-spe: Override attr->sample

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Date:   Mon, 17 Jan 2022 08:28:07 -0800
From:   Ian Rogers <irogers@...gle.com>
To:     German Gomez <german.gomez@....com>
Cc:     James Clark <james.clark@....com>, linux-kernel@...r.kernel.org,
        linux-perf-users@...r.kernel.org,
        Chase Conklin <chase.conklin@....com>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        Stephane Eranian <eranian@...gle.com>, netdev@...r.kernel.org,
        bpf@...r.kernel.org, "acme@...nel.org" <acme@...nel.org>
Subject: Re: [PATCH] perf record/arm-spe: Override attr->sample_period for
 non-libpfm4 events

On Mon, Jan 17, 2022 at 2:27 AM German Gomez <german.gomez@....com> wrote:
>
> Hi James,
>
> On 17/01/2022 09:59, James Clark wrote:
> >
> > On 14/01/2022 21:21, German Gomez wrote:
> >> A previous commit preventing attr->sample_period values from being
> >> overridden in pfm events changed a related behaviour in arm_spe.
> >>
> >> Before this patch:
> >> perf record -c 10000 -e arm_spe_0// -- sleep 1
> >>
> >> Would not yield an SPE event with period=10000, because the arm-spe code
> > Just to clarify, this seems like it should say "Would yield", not "Would not yield",
> > as in it was previously working?
>
> "this patch" refers to the patch I'm sending, not the one it's fixing.
> I might have to rewrite this to make it more clear. How about:
>
> ===
> A previous patch preventing "attr->sample_period" values from being
> overridden in pfm events changed a related behaviour in arm-spe.
>
> Before said patch:
> perf record -c 10000 -e arm_spe_0// -- sleep 1
>
> Would yield an SPE event with period=10000. After the patch, the period
> in "-c 10000" was being ignored because the arm-spe code initializes
> sample_period to a non-zero value.
>
> This patch restores the previous behaviour for non-libpfm4 events.
> ===

Thanks for fixing this, I can add an acked-by for the v2 patch. Could
we add a test for this to avoid future regressions? There are similar
tests for frequency like:
https://git.kernel.org/pub/scm/linux/kernel/git/acme/linux.git/tree/tools/perf/tests/attr/test-record-freq
based on the attr.py test:
https://git.kernel.org/pub/scm/linux/kernel/git/acme/linux.git/tree/tools/perf/tests/attr.py
The test specifies a base type of event attribute and then what is
modified by the test. It takes a little to get your head around but
having a test for this would be a welcome addition.

Thanks!
Ian

> Thanks for the review,
> German