linux-kernel - Re: [RFC PATCH V2 4/9] perf/x86: Enable post-processing monotonic raw conversion

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Date:   Tue, 14 Feb 2023 15:21:59 -0500
From:   "Liang, Kan" <kan.liang@...ux.intel.com>
To:     Thomas Gleixner <tglx@...utronix.de>, jstultz@...gle.com,
        peterz@...radead.org, mingo@...hat.com,
        linux-kernel@...r.kernel.org
Cc:     sboyd@...nel.org, eranian@...gle.com, namhyung@...nel.org,
        ak@...ux.intel.com, adrian.hunter@...el.com,
        Ravi Bangoria <ravi.bangoria@....com>
Subject: Re: [RFC PATCH V2 4/9] perf/x86: Enable post-processing monotonic raw
 conversion



On 2023-02-14 3:02 p.m., Thomas Gleixner wrote:
> Kan!
> 
> On Mon, Feb 13 2023 at 11:07, kan liang wrote:
>> From: Kan Liang <kan.liang@...ux.intel.com>
>> +	} else if (perf_event_hw_time(event)) {
>> +		struct ktime_conv mono;
>> +
>> +		userpg->cap_user_time_mono_raw = 1;
>> +		ktime_get_fast_mono_raw_conv(&mono);
> 
> What guarantees that the clocksource used by the timekeeping core is
> actually TSC? Nothing at all. You cannot make assumptions here.
> 

Yes, you are right.
I will add a check to make sure the clocksource is TSC when perf does
the conversion.

Could you please comment on whether the patch is in the right direction?
This V2 patch series expose the kernel internal conversion information
into the user space. Is it OK for you?

Thanks,
Kan