On Tue, Apr 26, 2022 at 11:02 AM Christian König christian.koenig@amd.com 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.
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.
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.