Re: [RFC 5/5] x86,perf: Only allow rdpmc if a perf_event is mapped

[Peter Zijlstra <peterz@infradead.org>]

On Sun, Oct 19, 2014 at 03:05:42PM -0700, Andy Lutomirski wrote:
> On Oct 19, 2014 2:33 PM, "Peter Zijlstra" <peterz@infradead.org> wrote:

> > > Also, I don't understand the purpose of cap_user_time.  Wouldn't it be
> > > easier to just record the last CLOCK_MONOTONIC time and let the user
> > > call __vdso_clock_gettime if they need an updated time?
> >
> > Because perf doesn't use CLOCK_MONOTONIC. Due to performance
> > considerations we used the sched_clock stuff, which tries its best to
> > make the best of the TSC without reverting to HPET and the like.
> >
> > Not to mention that CLOCK_MONOTONIC was not available from NMI context
> > until very recently.
> 
> I'm only talking about the userspace access to when an event was
> enabled and how long it's been running.  I think that's what the
> cap_user_time stuff is for.  I don't think those parameters are
> touched from NMI, right?
> 
> Point taken about sched_clock, though.

Well, mixing two time bases, one TSC based and one CLOCK_MONOTONIC is
just asking for trouble IMO ;-)

> > Also, things like c73deb6aecda ("perf/x86: Add ability to calculate TSC
> > from perf sample timestamps") seem to suggest people actually use TSC
> > for things as well.
> >
> > Now we might change to using the new NMI safe CLOCK_MONOTONIC (with a
> > fallback to use the sched_clock stuff on time challenged hardware) in
> > order to ease the correlation between other trace thingies, but even
> > then it makes sense to have this, having it here and reading the TSC
> > within the seqcount loop ensures you've got consistent data and touch
> > less cachelines for reading.
> 
> True.
> 
> OTOH, people (i.e. I) have optimized the crap out of
> __vdso_clock_gettime, and __vdso_perf_event_whatever could be
> similarly optimized.

Maybe, but at that point we commit to yet another ABI... I'd rather just
put a 'sane' implementation in a library or so.