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

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Message-ID: <43e59aee-dd94-0b95-25b2-dd6d14bd7c7a@amd.com>
Date:   Mon, 5 Jun 2023 15:33:15 +0530
From:   Ravi Bangoria <ravi.bangoria@....com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>, 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>,
        Ravi Bangoria <ravi.bangoria@....com>
Subject: Re: [REGRESSION][BISECT] perf/core: Remove pmu linear searching code

On 05-Jun-23 3:24 PM, Mark Rutland wrote:
> On Mon, Jun 05, 2023 at 03:04:45PM +0530, Ravi Bangoria wrote:
>> On 05-Jun-23 12:40 PM, Mark Rutland wrote:
>>> 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.
>>
>> Not sure I follow.
>>
>> PERF_TYPE_HARDWARE and PERF_TYPE_HW_CACHE are aliased to PERF_TYPE_RAW in
>> perf_init_event(). And PERF_TYPE_RAW should be present in pmu_idr if it
>> was registered using:
>>
>>   perf_pmu_register(pmu, "name", PERF_TYPE_RAW);
> 
> As I said, the PMUs get registered with a dynamic type, and there's no
> registration with PERF_TYPE_RAW. On arm/arm64 systems, *every* CPU PMU gets
> registered with:
> 
> 	perf_pmu_register(pmu, name, -1);
> 
> ... and *none* are registered with:
> 
> 	perf_pmu_register(pmu, name, PERF_TYPE_RAW)
> 
> ... so those all get a dynamic IDR type, and nothing gets placed into the IDR
> for PERF_TYPE_RAW, etc. We rely on the linear search to find a PMU that can
> handle PERF_TYPE_RAW, etc.

Got it. Thanks for the clarification.

Ravi