linux-kernel - Re: [PATCH v6 2/3]: perf/core: use context tstamp

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Date:   Fri, 4 Aug 2017 14:35:34 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alexey Budankov <alexey.budankov@...ux.intel.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Kan Liang <kan.liang@...el.com>,
        Dmitri Prokhorov <Dmitry.Prohorov@...el.com>,
        Valery Cherepennikov <valery.cherepennikov@...el.com>,
        Mark Rutland <mark.rutland@....com>,
        Stephane Eranian <eranian@...gle.com>,
        David Carrillo-Cisneros <davidcc@...gle.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6 2/3]: perf/core: use context tstamp_data for skipped
 events on mux interrupt

On Thu, Aug 03, 2017 at 09:47:56PM +0300, Alexey Budankov wrote:
> On 03.08.2017 18:00, Peter Zijlstra wrote:

> > Are the magic spots, right? And I'm not convinced its right.
> > 
> > Suppose I have two events in my context, and I created them 1 minute
> > apart. Then their respective tstamp_enabled are 1 minute apart as well.
> > But the above doesn't seem to preserve that difference.
> > 
> > A similar argument can be made for running I think. That is a per event
> > value and cannot be passed along to the ctx and back.
> 
> Aww, I see your point and it challenges my initial assumptions. 
> Let me think thru the case more. There must be some solution. Thanks!

So the sensible thing is probably to rewrite the entire time tracking to
make more sense. OTOH that's also the riskiest.

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;
	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;
	}
}

__read_curent_times(event, u64 *enabled, u64 *running)
{
	u64 delta, now = perf_event_time(event);

	delta = now - event->tstamp;

	*enabled = event->total_time_enabled;
	if (event->state >= STATE_INACTIVE)
		*enabled += delta;
	*running = event->total_time_running
	if (event->state == STATE_ACTIVE)
		*running += delta;
}

perhaps? That instantly solves the problem I think, because now we don't
need to update inactive events. But maybe I missed some, could you
verify?