linux-kernel - Re: [PATCH 01/11] perf/x86: Fix native_perf_sched_clock_from_tsc() with __sched_clock

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Message-ID: <30c756f4-4b4b-1b32-d8b6-c785a6cabe45@intel.com>
Date:   Wed, 9 Feb 2022 16:26:02 +0200
From:   Adrian Hunter <adrian.hunter@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...hat.com>, linux-kernel@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        kvm@...r.kernel.org, H Peter Anvin <hpa@...or.com>,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Leo Yan <leo.yan@...aro.org>
Subject: Re: [PATCH 01/11] perf/x86: Fix native_perf_sched_clock_from_tsc()
 with __sched_clock_offset

On 09/02/2022 14:54, Peter Zijlstra wrote:
> On Wed, Feb 09, 2022 at 10:49:19AM +0200, Adrian Hunter wrote:
>> native_perf_sched_clock_from_tsc() is used to produce a time value that can
>> be consistent with perf_clock().  Consequently, it should be adjusted by
>> __sched_clock_offset, the same as perf_clock() would be.
>>
>> Fixes: 698eff6355f735 ("sched/clock, x86/perf: Fix perf test tsc")
>> Signed-off-by: Adrian Hunter <adrian.hunter@...el.com>
>> ---
>>  arch/x86/kernel/tsc.c | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
>> index a698196377be..c1c73fe324cd 100644
>> --- a/arch/x86/kernel/tsc.c
>> +++ b/arch/x86/kernel/tsc.c
>> @@ -242,7 +242,8 @@ u64 native_sched_clock(void)
>>   */
>>  u64 native_sched_clock_from_tsc(u64 tsc)
>>  {
>> -	return cycles_2_ns(tsc);
>> +	return cycles_2_ns(tsc) +
>> +	       (sched_clock_stable() ? __sched_clock_offset : 0);
>>  }
> 
> Why do we care about the !sched_clock_stable() case?

I guess we don't.  So add __sched_clock_offset unconditionally then?