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

[Mark Rutland]

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:
> > On Tue, Jun 27, 2017 at 09:51:00PM -0700, Kyle Huey wrote:
> >> My understanding of the situation is as follows:
> >>
> >> There is some time, call it t_0, where the hardware counter overflows.
> >> The PMU triggers an interrupt, but this is not instantaneous.  Call
> >> the time when the interrupt is actually delivered t_1.  Then t_1 - t_0
> >> is the "skid".
> >>
> >> Note that if the counter is `exclude_kernel`, then at t_0 the CPU
> >> *must* be running a userspace program.  But by t_1, the CPU may be
> >> doing something else.  Your patch changed things so that if at t_1 the
> >> CPU is in the kernel, then the interrupt is discarded.  But rr has
> >> programmed the counter to deliver a signal on overflow (via F_SETSIG
> >> on the fd returned by perf_event_open).  This change results in the
> >> signal never being delivered, because the interrupt was ignored.
> >> (More accurately, the signal is delivered the *next* time the counter
> >> overflows, which is far past where we wanted to inject our
> >> asynchronous event into our tracee.
> >
> > Yes, this is a bug.
> >
> > As we're trying to avoid smapling state, I think we can move the check
> > into perf_prepare_sample() or __perf_event_output(), where that state is
> > actually sampled. I'll take a look at that momentarily.
> >
> > 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?

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.

> > Is that signal delivered to the tracee, or to a different process that
> > traces it? If the latter, what ensures that the task is stopped
> > sufficiently quickly?
> 
> It's delivered to the tracee (via an F_SETOWN_EX with the tracee tid).
> In practice we've found that on modern Intel hardware that the
> interrupt and resulting signal delivery delay is bounded by a
> relatively small number of counter events.

Ok.

Thanks,
Mark.