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

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Message-ID: <YgO5f8DFd2onnTOE@hirez.programming.kicks-ass.net>
Date:   Wed, 9 Feb 2022 13:54:23 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Adrian Hunter <adrian.hunter@...el.com>
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 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?