Re: [PATCH v2 07/54] perf: Add generic exclude_guest support

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On 2024-06-12 7:17 a.m., Peter Zijlstra wrote:
> On Tue, Jun 11, 2024 at 09:27:46AM -0400, Liang, Kan wrote:
>> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
>> index dd4920bf3d1b..68c8b93c4e5c 100644
>> --- a/include/linux/perf_event.h
>> +++ b/include/linux/perf_event.h
>> @@ -945,6 +945,7 @@ struct perf_event_context {
>>  	u64				time;
>>  	u64				timestamp;
>>  	u64				timeoffset;
>> +	u64				timeguest;
>>
>>  	/*
>>  	 * These fields let us detect when two contexts have both
> 
>> @@ -651,10 +653,26 @@ __perf_update_times(struct perf_event *event, u64
>> now, u64 *enabled, u64 *runnin
>>
>>  static void perf_event_update_time(struct perf_event *event)
>>  {
>> -	u64 now = perf_event_time(event);
>> +	u64 now;
>> +
>> +	/* Never count the time of an active guest into an exclude_guest event. */
>> +	if (event->ctx->timeguest &&
>> +	    event->pmu->capabilities & PERF_PMU_CAP_PASSTHROUGH_VPMU) {
>> +		/*
>> +		 * If a guest is running, use the timestamp while entering the guest.
>> +		 * If the guest is leaving, reset the event timestamp.
>> +		 */
>> +		if (__this_cpu_read(perf_in_guest))
>> +			event->tstamp = event->ctx->timeguest;
>> +		else
>> +			event->tstamp = event->ctx->time;
>> +		return;
>> +	}
>>
>> +	now = perf_event_time(event);
>>  	__perf_update_times(event, now, &event->total_time_enabled,
>>  					&event->total_time_running);
>> +
>>  	event->tstamp = now;
>>  }
> 
> So I really don't like this much, 

An alternative way I can imagine may maintain a dedicated timeline for
the PASSTHROUGH PMUs. For that, we probably need two new timelines for
the normal events and the cgroup events. That sounds too complex.

> and AFAICT this is broken. At the very
> least this doesn't work right for cgroup events, because they have their
> own timeline.

I think we just need a new start time for an event. So we may use
perf_event_time() to replace the ctx->time for the cgroup events.

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 019c237dd456..6c46699c6752 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -665,7 +665,7 @@ static void perf_event_update_time(struct perf_event
*event)
 		if (__this_cpu_read(perf_in_guest))
 			event->tstamp = event->ctx->timeguest;
 		else
-			event->tstamp = event->ctx->time;
+			event->tstamp = perf_event_time(event);
 		return;
 	}


> 
> Let me have a poke...

Sure.

Thanks,
Kan




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