Re: [PATCH v4.16-rc4 2/2] x86/vdso: on Intel, VDSO should handle CLOCK_MONOTONIC_RAW

Peter Zijlstra
2018-03-14T13:16:36Z

On Wed, Mar 14, 2018 at 12:55:20PM +0000, Jason Vas Dias wrote:
> > If you want to correlate to CLOCK_MONOTONIC_RAW you have to read
> > CLOCK_MONOTONIC_RAW and not some random other clock value.
> >
>
> Exactly ! Hence the need for the patch so that users can get
> CLOCK_MONOTONIC_RAW values with low latency and correlate them
> with PERF CPU_CLOCK values.

No, you _CANNOT_ correlate CLOCK_MONOTONIC_RAW with CPU_CLOCK, that is
_BROKEN_.

Yes it 'works', but that's mostly a happy accident. There is no
guarantee that CLOCK_MONOTONIC_RAW runs off the TSC, and even if both
CPU_CLOCK and CLOCK_MONOTONIC_RAW use the TSC, they need not use the
same rate (and they didn't for a long time).

Do not mix clocks.