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.