Re: [PATCH v6 2/3]: perf/core: use context tstamp_data for skipped events on mux interrupt

[Peter Zijlstra]

On Fri, Aug 04, 2017 at 02:35:34PM +0200, Peter Zijlstra wrote:
> Something like:
> 
> __update_state_and_time(event, new_state)
> {
>       u64 delta, now = perf_event_time(event);
>       int old_state = event->state;
> 
>       event->tstamp = now;
>       event->state  = new_state;
> 
>       delta = now - event->tstamp;
Obv should go above the tstamp assignment

>       switch (state) {
>       case STATE_ACTIVE:
>               WARN_ON_ONCE(old_state != STATE_INACTIVE);
>               event->total_time_enabled += delta;
>               break;
> 
>       case STATE_INACTIVE:
>               switch (old_state) {
>               case STATE_OFF:
>                       /* ignore the OFF -> INACTIVE period */
>                       break;
> 
>               case STATE_ACTIVE:
>                       event->total_time_enabled += delta;
>                       event->total_time_running += delta;
>                       break;
> 
>               default:
>                       WARN_ONCE();
>               }
>               break;
> 
>       case STATE_OFF:
>               WARN_ON_ONCE(old_state != STATE_INACTIVE)
>               event->total_time_enabled += delta;
>               break;
>       }
> }

So that's a straight fwd state machine that deals with:

  OFF <-> INACTIVE <-> ACTIVE

but I think something like:

__update_state_and_time(event, new_state)
{
        u64 delta, new = perf_event_time(event);
        int old_state = event->state;

        delta = now - event->tstamp;
        event->tstamp = now;
        event->state  = new_state;

        if (old_state == STATE_OFF)
                return;

        event->total_time_enabled += delta;

        if (old_state == STATE_ACTIVE)
                event->total_time_running += delta;
}

is equivalent and generates smaller code.. but again, double check (also
it doesn't validate the state transitions).