linux-kernel - Re: [REGRESSION][BISECT] perf/core: Remove pmu linear searching code

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Message-ID: <ZH2KVyyC5oMr+Vk2@FVFF77S0Q05N>
Date:   Mon, 5 Jun 2023 08:10:15 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        Ravi Bangoria <ravi.bangoria@....com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>
Cc:     jolsa@...nel.org, irogers@...gle.com, bp@...en8.de,
        adrian.hunter@...el.com, linux-perf-users@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        "linux-samsung-soc@...r.kernel.org" 
        <linux-samsung-soc@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "regressions@...ts.linux.dev" <regressions@...ts.linux.dev>
Subject: Re: [REGRESSION][BISECT] perf/core: Remove pmu linear searching code

On Sun, Jun 04, 2023 at 01:38:10PM +0200, Krzysztof Kozlowski wrote:
> Hi,
> 
> #regzbot introduced: 9551fbb64d09
> 
> Bisect pointed to commit 9551fbb64d09 ("perf/core: Remove pmu linear
> searching code") as first one where all hardware events are gone from
> perf for ARMv7 Exynos5422 board.

I think that commit 9551fbb64d09 is just wrong.

The commit message asserts:

  Searching for the right pmu by iterating over all pmus is no longer
  required since all pmus now *must* be present in the 'pmu_idr' list.
  So, remove linear searching code.

... and while each PMU has *some* entry in the pmu_idr list, for its dynamic
type, that means that events with other types (e.g. PERF_TYPE_HARDWARE or
PERF_TYPE_RAW) will fail to find a PMU in the IDR whereas they'd previously
have been accepted by a PMU during the subsequent iteration over all PMUs.

So that iteration is still required.

How does this work on x86? Do you have pseudo-PMUs for PERF_TYPE_HARDWARE and
PERF_TYPE_RAW ?

Thanks,
Mark.

> Expected:
> perf list | grep "Hardware event"
>   branch-instructions OR branches                    [Hardware event]
>   branch-misses                                      [Hardware event]
>   bus-cycles                                         [Hardware event]
>   cache-misses                                       [Hardware event]
>   cache-references                                   [Hardware event]
>   cpu-cycles OR cycles                               [Hardware event]
>   instructions                                       [Hardware event]
> 
> 
> Result:
> empty, no hardware events
> 
> 1. Arch ARM Linux
> 2. exynos_defconfig
> 3. Board:
>    - Odroid HC1
>      ARMv7, octa-core (Cortex-A7+A15), Exynos5422 SoC
>      Systemd: v239
> 
> 4. perf version 6.1.0
> 
> 
> Best regards,
> Krzysztof