Re: [RFC] perf: need to expose sched_clock to correlate user sampleswith kernel samples

[Thomas Gleixner]

On Tue, 19 Feb 2013, John Stultz wrote:
> On 02/18/2013 12:35 PM, Thomas Gleixner wrote:
> > On Tue, 5 Feb 2013, John Stultz wrote:
> > > On 02/05/2013 02:13 PM, Stephane Eranian wrote:
> > > > But if people are strongly opposed to the clock_gettime() approach, then
> > > > I can go with the ioctl() because the functionality is definitively
> > > > needed
> > > > ASAP.
> > > I prefer the ioctl method, since its less likely to be
> > > re-purposed/misused.
> > Urgh. No! With a dedicated CLOCK_PERF we might have a decent chance to
> > put this into a vsyscall. With an ioctl not so much.
> >   
> > > Though I'd be most comfortable with finding some way for perf-timestamps
> > > to be
> > > CLOCK_MONOTONIC based (or maybe CLOCK_MONOTONIC_RAW if it would be
> > > easier),
> > > and just avoid all together adding another time domain that doesn't really
> > > have clear definition (other then "what perf uses").
> > What's wrong with that. We already have the infrastructure to create
> > dynamic time domains which can be completely disconnected from
> > everything else.
> 
> Right, but those are for actual hardware domains that we had no other way of
> interacting with.

Agreed.

> > Tracing/perf/instrumentation is a different domain and the main issue
> > there is performance. So going for a vsyscall enabled clock_gettime()
> > approach is definitely the best thing to do.
> 
> So describe how the perf time domain is different then CLOCK_MONOTONIC_RAW.

Mostly not, except for x86 :)
 
> My concern here is that we're basically creating a kernel interface that
> exports implementation-defined semantics (again: whatever perf does right
> now). And I think folks want to do this, because adding CLOCK_PERF is easier
> then trying to:
> 
> 1) Get a lock-free method for accessing CLOCK_MONOTONIC_RAW

Well, you can't. We already guarantee that CLOCK_MONOTONIC_RAW is
monotonic and we can't break that when adding a vsyscall
implementation. So the gtod->seq or any equivalent needs to stay, no
matter what.

OTOH, thinking more about it:

If you look at the dance sched_clock_local() is doing, it can be
doubted that a vsyscall based access to CLOCK_MONOTONIC_RAW is going
to be massivly more overhead.

The periodic update of sched_clock will cause the same issues as the
gtod->seq write hold time, which is extremly short. It's not a
seqcount in sched_clock_local(), it's a cmpxchg based retry loop.

Would be interesting to compare and contrast that. Though you can't do
that in the kernel as the write hold time of the timekeeper seq is way
larger than the gtod->seq write hold time. I have a patch series in
work which makes the timekeeper seq hold time almost as short as that
of gtod->seq.

Even if sched_clock is stable, the overhead of dealing with short
counters or calling mult_fract() is probably in the same range as what
we do with the timekeeping related vsyscalls.

> 2) Having perf interpolate its timestamps to CLOCK_MONOTONIC, or
> CLOCKMONOTONIC_RAW when it exports the data

If we can get the timekeeper seq write hold time down to the bare
minimum (comparable to gtod->seq) I doubt that sched_clock will have
any reasonable advantage.
 
> The semantics on sched_clock() have been very flexible and hand-wavy in the
> past. And I agree with the need for the kernel to have a "fast-and-loose"
> clock as well as the benefits to that flexibility as the scheduler code has
> evolved.  But non-the-less, the changes in its semantics have bitten us badly
> a few times.
> 
> So I totally understand why the vsyscall is attractive. I'm just very cautious
> about exporting a similarly fuzzily defined interface to userland. So until
> its clear what the semantics will need to be going forward (forever!), my
> preference will be that we not add it.

Fair enough.

Thanks,

	tglx
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