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.