Messages in this thread | | | Date | Wed, 14 Mar 2018 14:16:26 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH v4.16-rc4 2/2] x86/vdso: on Intel, VDSO should handle CLOCK_MONOTONIC_RAW |
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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.
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