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

[Peter Zijlstra]

On Thu, Feb 19, 2015 at 04:38:57PM +0200, Adrian Hunter wrote:
> 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)

which is slew adjustment; read the adjtime manpage :-)

> > And NTP limits the slew rate to 500 PPM, so even if you would get a
> 
> Assuming it is not broken.

NTP people are a cautious crowd, sure they get it wrong just like the
rest of us, but mostly it needs to work.

> > 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.

What decoder? perf report is already subject to much larger shifts in
time if you run it on say a core2 machine.

> > 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.

Uhm what? Userspace is already awake.

> > 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.

Finding multiple samples and interpolating between them is much simpler
than reading tsc and doing the mult, shift and offset addition?

I suspect you're talking about something else entirely; your changelogs
are inadequate for they tell ntohing of your usecase and have me
guessing. Don't do that.
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