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

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Date:   Tue, 11 Jul 2023 15:15:55 +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 4/4] perf: Remove unused PERF_PMU_CAP_HETEROGENEOUS_CPUS
 capability



On 11/07/2023 13:10, 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 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>
>> ---
>>  include/linux/perf_event.h | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
>> index d5628a7b5eaa..3f4d941fd6c5 100644
>> --- a/include/linux/perf_event.h
>> +++ b/include/linux/perf_event.h
>> @@ -288,7 +288,7 @@ struct perf_event_pmu_context;
>>  #define PERF_PMU_CAP_EXTENDED_REGS		0x0008
>>  #define PERF_PMU_CAP_EXCLUSIVE			0x0010
>>  #define PERF_PMU_CAP_ITRACE			0x0020
>> -#define PERF_PMU_CAP_HETEROGENEOUS_CPUS		0x0040
>> +/* Unused					0x0040 */
> 
> Small nit, "Unused" marking might not be required here.
> 

But then it would be very easy to miss that there is a free bit if I
don't leave the comment. Is it really better without it?

I could shift all the following ones down by one bit, but it would be a
lot of work to make sure that nobody has hard coded some check for one
of the bits instead of using the define somewhere.

>>  #define PERF_PMU_CAP_NO_EXCLUDE			0x0080
>>  #define PERF_PMU_CAP_AUX_OUTPUT			0x0100
>>  #define PERF_PMU_CAP_EXTENDED_HW_TYPE		0x0200