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

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Date:   Fri, 4 Aug 2017 17:23:35 +0300
From:   Alexey Budankov <alexey.budankov@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
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 04.08.2017 15:35, Peter Zijlstra wrote:
> 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?

Thanks for the input. I will check it.

> 
>