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

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Date:   Fri, 4 Aug 2017 17:25:55 +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 15:51, Peter Zijlstra wrote:
> 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).

Accepted.

>