Re: [RFC 00/14] i915 PMU and engine busy stats

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 19/07/2017 13:05, Chris Wilson wrote:
> Quoting Tvrtko Ursulin (2017-07-18 15:36:04)
>> From: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>>
>> Rough sketch of the idea I mentioned a few times to various people - merging
>> the engine busyness tracking with Chris i915 PMU RFC.
>>
>> First patch is the actual PMU RFC by Chris. It is followed by some cleanup
>> patches, then come a few improvements, cheap execlists engine busyness tracking,
>> debugfs view for the same, and finally the i915 PMU is extended to use this
>> instead of timer based mmio sampling.
>>
>> This makes it cheaper and also more accurate since engine busyness is not
>> derived via sampling.
>>
>> But I haven't figure out the perf API yet. For example is it possible to access
>> our events in an usable fashion via perf top/stat or something? Do we want to
>> make the events discoverable as I did (patch 8).
> 
> In my dreams I have gpu activity in the same perf timechart as gpu
> activity. But that can be mostly by the request tracepoints, but still
> overlaying cpu/gpu activity is desirable and more importantly we want to
> coordinate with nouveau/amdgpu so that such interfaces are as agnostic
> as possible. There are definitely a bunch of global features in common
> for all (engine enumeration & activity, mempool enumeration, size &
> activty, power usage?). But the key question is how do we build for the
> future? Split the event id range into common/driver?

I don't know if going for common events would be workable. A few metrics 
sounds like it could be generic, but I am not sure there would be more 
than a couple where that would be future proof. Also is the coordination 
effort (no one else seems to implement a perf interface at the moment) 
worth it at the current time? I am not sure.

>> I could not find much (any?) kernel API level documentation for perf.
> 
> There isn't much indeed. Given that we now have a second pair of eyes go
> over the sampling and improve its interaction with i915, we should start
> getting PeterZ involved to check the interaction with perf.

Okay, I guess another cleanup pass and then I can do that.

In the meantime do you have any good understanding of what kind of 
events are we exposing here? They look weird if I record them and look 
with "perf script", and "perf stat" always reports zeroes for them. But 
they still work from the overlay tool. So it is a bit of a mystery to me 
what they really are.
>> Btw patch series actually works since intel-gpu-overlay can use these events
>> when they are available.
>>
>> Chris Wilson (1):
>>    RFC drm/i915: Expose a PMU interface for perf queries
> 
> One thing I would like is for any future interface (including this
> engine/class/event id) to use the engine class/instance mapping.

I was thinking about that myself. I can do it in the next cleanup pass.

Regards,

Tvrtko
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