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Date: Wed, 14 Mar 2018 14:16:26 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Jason Vas Dias <jason.vas.dias@gmail.com>
Cc: x86@kernel.org, LKML <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>, andi <andi@firstfloor.org>
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.