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

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Date:   Wed, 14 Mar 2018 14:16:26 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Jason Vas Dias <jason.vas.dias@...il.com>
Cc:     x86@...nel.org, LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        andi <andi@...stfloor.org>
Subject: Re: [PATCH v4.16-rc4 2/2] x86/vdso: on Intel, VDSO should handle
 CLOCK_MONOTONIC_RAW

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.