Am 26.04.22 um 20:50 schrieb Chia-I Wu:
On Tue, Apr 26, 2022 at 11:02 AM Christian König
<christian.koenig@xxxxxxx> wrote:
Am 26.04.22 um 19:40 schrieb Chia-I Wu:
[SNIP]
Well I just send a patch to completely remove the trace point.
As I said it absolutely doesn't make sense to use this for
visualization, that's what the trace_dma_fence_init trace point is good for.
I am a bit confused by this. _emit and _signaled are a great way to
see how many fences are pending from cpu's point of view. How does
_emit make no sense and _init is good instead?
We had exactly that confusion now multiple times and it's one of the
main reasons why I want to remove the _emit trace point.
See the when you want to know how many fences are pending you need to
watch out for init/destroy and *NOT* emit.
The reason is that in the special case where emit makes sense (e.g. the
GPU scheduler fences) emit comes later than init, but pending on the CPU
and taking up resources are all fences and not just the one emitted to
the hardware.
I am more interested in pending on the GPU.
On the other hand when you want to measure how much time each operation
took on the hardware you need to take a look at the differences of the
signal events on each timeline.
_signaled alone is not enough when the GPU is not always busy. After
the last pending fence signals but before the following _init/_emit,
nothing is pending.
Yeah, I'm perfectly aware of that.
For all drivers except virtio-gpu, _init and "ring head update" always
happen close enough that I can see why _emit is redundant. But I like
having _emit as a generic tracepoint for timelines where _init and
_emit can be apart, instead of requiring a special case tracepoint for
each special case timeline.
And I'm certainly not going to add _emit to all drivers just because of
that. As you said it is a special case for virtio-gpu and the GPU scheduler.
And as I explained before the difference between _init and _emit
shouldn't matter to your visualization. The background is that as soon
as an dma_fence is initialized with _init it is "live" regarding the
dependency and memory management and exactly that's what matters for the
visualization.
The latency between _init and _emit is just interesting for debugging
the scheduler and surprisingly virtio-gpu as well, for all other use
cases it is irrelevant.
Regards,
Christian.
So there isn't really any use case for the emit trace point, except when
you want to figure out how much latency the scheduler introduce. Then
you want to take a look at init and emit, but that isn't really that
interesting for performance analyses.
Regards,
Christian.
