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

[Adrian Hunter]

On 19/02/2015 5:05 p.m., Peter Zijlstra wrote:
> 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.

Any decoder of Intel PT data. Side-band events like sched_switch or mmap
have to be sync'ed with Intel PT TSC timestamps to decode the trace. But
synchronizing any kind of event could be useful for analysis.

> > > 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.

For Intel PT recording, perf record will be sleeping on poll().

> > > 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.

Sorry. I did mention Intel PT in patch 2, but I basically assumed the
need to synchronize events with other time sources was understood.
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