Re: [REGRESSION] perf/core: PMU interrupts dropped if we entered the kernel in the "skid" region

[Mark Rutland]

On Wed, Jun 28, 2017 at 10:36:20AM -0700, Kyle Huey wrote:
> On Wed, Jun 28, 2017 at 10:19 AM, Mark Rutland <mark.rutland@xxxxxxx> wrote:
> > On Wed, Jun 28, 2017 at 09:46:43AM -0700, Kyle Huey wrote:
> >> On Wed, Jun 28, 2017 at 3:12 AM, Mark Rutland <mark.rutland@xxxxxxx> wrote:

> >> > Just to clarify, you don't care about the sample state at all? i.e. you
> >> > don't need the user program counter?
> >>
> >> Right. `sample_regs_user`, `sample_star_user`, `branch_sample_type`,
> >> etc are all 0.
> >> https://github.com/mozilla/rr/blob/cf594dd01f07d96a61409e9f41a29f78c8c51693/src/PerfCounters.cc#L194
> >> is what we do use.
> >
> > Given that, I must be missing something.
> >
> > In __perf_event_overflow(), we already bail out early if
> > !is_sampling_event(event), i.e. when the sample_period is 0.
> >
> > Your attr has a sample_period of zero, so something must be initialising
> > that.
> >
> > Do you always call PERF_EVENT_IOC_PERIOD, or is something in the core
> > fiddling with the sample period behind your back?
> 
> We always either set sample_period or call PERF_EVENT_IOC_PERIOD (with
> an enormous number if we don't actually want an interrupt.  See
> `PerfCounters::reset`, line 446.

Ah, thanks for the pointer.

> > It seems odd that an event without any samples to take has a sample
> > period. I'm surprised that there's not *some* sample_type set.
> 
> Perhaps sample_period is misleadingly named :)  Alternatively, you
> could imagine it as sampling where we're only interested in whether
> the counter passed the sampling value or not.

Sure; it's just that I suspect the existing kernel behviour isn't
*quite* intentional, and I could easily see it getting broken in future,
e.g. if someone were to make is_sampling_event() check the attr for
sample types.

So we need to keep an eye on that, regardless.

Thanks,
Mark.