On Wed, Sep 25, 2013 at 12:12:16PM -0700, Andi Kleen wrote: > On Wed, Sep 25, 2013 at 02:50:26PM +0200, Jiri Olsa wrote: > > hi, > > sending *RFC* for toggling events support. > > > > Adding perf interface that allows to create toggle events, which can > > enable or disable another event. Whenever the toggle event is triggered > > (has overflow), it toggles another event state and either starts or > > stops it. > > > > The goal is to be able to create toggling tracepoint events to enable and > > disable HW counters, but the interface is generic enough to be used for > > any kind of event. > > Haven't read the patches, but frequent full event switch in/out seems > very expensive. If someone puts that switch on a common > function it would likely disturb things quite a bit.
We dont do full sched in/out.. the toggled event is scheduled in 'paused' state which means that it's not started. Once the trigger is hit, pmu start/stop is executed. > > It would be better to keep counting and just do RDPMC on > the switch points, and then subtract for counting. > For sampling could need a MSR write to enable/disable. > Still somewhat expensive, but nowhere near as bad as a full switch. I'll check on that > > Another problem is that it may be very inexact, as > the counting will often happen in the background > and not be very synchronized with the switches. > Not fully sure how big a problem that would be. the toggling overflow function does following (perf_event_toggle_overflow) - disable pmu of the toggled event - start/stop the toggle event - store sample for the trigger function - enable pmu of the toggled event so the overhead (extra count) is: - the return code from pmu enable and return from the overflow processing - trigger event overflow processing till pmu disable code and no overhead for user space events ;-) thanks, jirka -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
