linux-kernel - Re: [PATCH 3/4] arm_pmu: Remove unused PERF_PMU_CAP_HETEROGENEOUS

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Date:   Tue, 11 Jul 2023 15:04:54 +0100
From:   James Clark <james.clark@....com>
To:     Anshuman Khandual <anshuman.khandual@....com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, Will Deacon <will@...nel.org>,
        Kan Liang <kan.liang@...ux.intel.com>,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-perf-users@...r.kernel.org, irogers@...gle.com
Subject: Re: [PATCH 3/4] arm_pmu: Remove unused
 PERF_PMU_CAP_HETEROGENEOUS_CPUS capability



On 11/07/2023 13:08, Anshuman Khandual wrote:
> 
> 
> On 7/10/23 17:51, James Clark wrote:
>> Since commit bd2756811766 ("perf: Rewrite core context handling") the
>> relationship between perf_event_context and PMUs has changed so that
>> the error scenario that PERF_PMU_CAP_HETEROGENEOUS_CPUS originally
>> silenced no longer exists.
>>
>> Remove the capability and associated comment to avoid confusion that it
>> actually influences any perf core behavior. This change should be a
>> no-op.
>>
>> Signed-off-by: James Clark <james.clark@....com>
>> ---
>>  drivers/perf/arm_pmu.c | 7 ++-----
>>  1 file changed, 2 insertions(+), 5 deletions(-)
>>
>> diff --git a/drivers/perf/arm_pmu.c b/drivers/perf/arm_pmu.c
>> index d8844a9461a2..297906df6628 100644
>> --- a/drivers/perf/arm_pmu.c
>> +++ b/drivers/perf/arm_pmu.c
>> @@ -872,15 +872,12 @@ struct arm_pmu *armpmu_alloc(void)
>>  		.attr_groups	= pmu->attr_groups,
>>  		/*
>>  		 * This is a CPU PMU potentially in a heterogeneous
>> -		 * configuration (e.g. big.LITTLE). This is not an uncore PMU,
>> -		 * and we have taken ctx sharing into account (e.g. with our
>> -		 * pmu::filter callback and pmu::event_init group validation).
>> -		 *
>> +		 * configuration (e.g. big.LITTLE) so
>>  		 * PERF_PMU_CAP_EXTENDED_HW_TYPE is required to open the legacy
>>  		 * PERF_TYPE_HARDWARE and PERF_TYPE_HW_CACHE events on a
>>  		 * specific PMU.
>>  		 */
>> -		.capabilities	= PERF_PMU_CAP_HETEROGENEOUS_CPUS | PERF_PMU_CAP_EXTENDED_REGS |
>> +		.capabilities	= PERF_PMU_CAP_EXTENDED_REGS |
>>  				  PERF_PMU_CAP_EXTENDED_HW_TYPE,
> 
> Small nit, the second line could be folded back into the first one.

.clang-format has a column limit of 80 which is why it gets folded. The
kernel coding style guide also still says 80 is a strongly preferred
limit so I'll probably leave it.

> 
>>  	};
>>