Re: [PATCH] perf/x86/intel/ds: Fix the conversion from TSC to perf time

[Liang, Kan]

On 2023-01-23 3:39 p.m., Peter Zijlstra wrote:
> On Mon, Jan 23, 2023 at 09:20:27AM -0800, kan.liang@xxxxxxxxxxxxxxx wrote:
>> The TSC unstable case seems to be some corner cases (e.g., due to broken
>> BIOS). This patch doesn't support the conversion when the TSC is
>> unstable. The TSC in a PEBS record will be dropped and fallback to the
>> software perf time provided by the generic code.
> 
> :-(
> 
> You're saying there's modern systems (PEBS timestamps are fairly new)
> that trigger unstable TSC ?
> 
> What systems in specific have you observed this on -- we really need to
> name and shame them, this is fully unacceptable.

No, I didn't observe any systems which trigger unstable TSC.

Here, I tried to explain the reason why the patch didn't support the
conversion when the TSC is unstable. Because it's a corner case and very
unlikely to happen.
The "broken BIOS" case is quoted from the __sched_clock_work(), which
shows that the broken BIOS may be a reason of unstable TSC.

Sorry for the confusion caused.

Let me rephrase this.
"This patch doesn't support the conversion when the TSC is unstable.
Because the TSC unstable case is a corner case and very unlikely to
happen. Even if it happens, the TSC in a PEBS record will be dropped and
fall back to the inaccurate software perf time provided by the generic
code as perf does it on the previous platforms."

Thanks,
Kan