Re: [PATCH 0/2] perf/x86: Add ability to sample TSC

From: Adrian Hunter
Date: Thu Feb 19 2015 - 09:41:22 EST

On 19/02/15 15:50, Peter Zijlstra wrote:
> On Thu, Feb 19, 2015 at 02:11:08PM +0200, Adrian Hunter wrote:
>> Hi
>> With the advent of switching perf_clock to CLOCK_MONOTONIC,
>> it will not be possible to convert perf_clock directly to/from
>> TSC. So add the ability to sample TSC instead.
> Well, you can, mostly. MONOTONIC is only affected by NTP slew rate
> changes, not offset changes.

man page says is also subject to adjtime(3)

> And NTP limits the slew rate to 500 PPM, so even if you would get a

Assuming it is not broken.

> slew change and then not update the userpage data for a second you'd be
> maximally off by 0.0005 seconds.

That could still be enough to break the decoder. It will certainly
misrepresent the order of events, which is a big loss of information.

> And that is way below what the current perf clock guarantees on funny
> hardware.
> If you're really worried about this; we could maybe get John and Thomas
> to allow us a callback on every slew change so we can update the
> userpage data ASAP, much reducing the max error.
> Say it takes a 10e5 cycles to update your userpage, then you're never
> further off than 50 cycles, which is below your ART multiplier.

You still need to wake up user space to read the userpage.

> Does that really matter? Also, if you have a stable crystal, the slew
> rate change should be minimal and infrequent, never getting you close to
> these numbers.
> So no, I'm not convinced we need this.

Adding TSC to the sample is a lot simpler and more accurate.

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