On Mon, May 22, 2017 at 12:28:26PM -0700, Stephane Eranian wrote: > On Mon, May 22, 2017 at 12:23 PM, Peter Zijlstra <pet...@infradead.org> wrote: > > On Mon, May 22, 2017 at 04:55:47PM +0000, Liang, Kan wrote: > >> > >> > >> > On Fri, May 19, 2017 at 10:06:21AM -0700, kan.li...@intel.com wrote: > >> > > diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c index > >> > > 580b60f..e8b2326 100644 > >> > > --- a/arch/x86/events/core.c > >> > > +++ b/arch/x86/events/core.c > >> > > @@ -101,6 +101,10 @@ u64 x86_perf_event_update(struct perf_event > >> > *event) > >> > > delta = (new_raw_count << shift) - (prev_raw_count << shift); > >> > > delta >>= shift; > >> > > > >> > > + /* Correct the count number if applying ref_cycles replacement */ > >> > > + if (!is_sampling_event(event) && > >> > > + (hwc->flags & PERF_X86_EVENT_REF_CYCLES_REP)) > >> > > + delta *= x86_pmu.ref_cycles_factor; > >> > > >> > That condition seems wrong, why only correct for !sampling events? > >> > > >> > >> For sampling, it's either fixed freq mode or fixed period mode. > >> - In the fixed freq mode, we should do nothing, because the adaptive > >> frequency algorithm will handle it. > >> - In the fixed period mode, we have already adjusted the period in > >> ref_cycles_rep(). > >> Therefore, we should only handle !sampling events here. > > > > How so? For sampling events the actual event count should also be > > accurate. > > Yes, it must be. Because you can reconstruct the total number of > occurrences of the event by adding > all the periods recorded in each sample. So the period in each sample > must reflect user event and not > kernel event.
Well, that, but you can equally use read() or the mmap()'ed rdpmc stuff on a sampling event. The fact that is also generates samples does not mean it should not also function as a non-sampling event.
