[Intel-xe] [PATCH 1/3] drm/suballoc: Introduce a generic suballocation manager

[Thomas Hellström]

Hi, Christian,

So I resurrected Maarten's previous patch series around this (the amdgpu 
suballocator) slightly modified the code to match the API of this patch 
series, re-introduced the per-allocation alignment as per a previous 
review comment from you on that series, and made checkpatch.pl pass 
mostly, except for pre-existing style problems, and added / fixed some 
comments. No memory corruption seen so far on limited Xe testing.

To move this forward I suggest starting with that as a common drm 
suballocator. I'll post the series later today. We can follow up with 
potential simplifactions lif needed.

I also made a kunit test also reporting some timing information. Will 
post that as a follow up. Some interesting preliminary conclusions:

* drm_mm is per se not a cpu hog, If the rb tree processing is disabled 
and the EVICT algorithm is changed from MRU to ring-like LRU traversal, 
it's more or less just as fast as the ring suballocator.

* With a single ring, and the suballocation buffer never completely 
filled (no sleeps) the amd suballocator is a bit faster per allocation / 
free. (Around 250 ns instead of 350). Allocation is slightly slower on 
the amdgpu one, freeing is faster, mostly due to the locking overhead 
incurred when setting up the fence callbacks, and for avoiding 
irq-disabled processing on the one I proposed.

* With multiple rings and varying allocation sizes and signalling times 
creating fragmentation, the picture becomes different as the amdgpu 
allocator starts to sleep/throttle already round 50% - 75% fill. The one 
I proposed between 75% to 90% fill, and once that happens, the CPU cost 
of putting to sleep and waking up should really shadow the above numbers.

So it's really a tradeoff. Where IMO also code size and maintainability 
should play a role.

Also I looked at the history of the amdgpu allocator originating back to 
Radeon 2012-ish, but couldn't find any commits mentioning fence 
callbacks nor problem with those. Could you point me to that discussion?

Thanks,

Thomas



On 2/17/23 14:51, Thomas Hellström wrote:
>
> On 2/17/23 14:18, Christian König wrote:
>> Am 17.02.23 um 14:10 schrieb Thomas Hellström:
>>> [SNIP]
>>>>>>>
>>>>>>> Any chance you could do a quick performance comparison? If not, 
>>>>>>> anything against merging this without the amd / radeon changes 
>>>>>>> until we can land a simpler allocator?
>>>>>>
>>>>>> Only if you can stick the allocator inside Xe and not drm, cause 
>>>>>> this seems to be for a different use case than the allocators 
>>>>>> inside radeon/amdgpu.
>>>>>
>>>>> Hmm. No It's allocating in a ring-like fashion as well. Let me put 
>>>>> together a unit test for benchmaking. I think it would be a 
>>>>> failure for the community to end up with three separate 
>>>>> suballocators doing the exact same thing for the same problem, 
>>>>> really.
>>>>
>>>> Well exactly that's the point. Those allocators aren't the same 
>>>> because they handle different problems.
>>>>
>>>> The allocator in radeon is simpler because it only had to deal with 
>>>> a limited number of fence timelines. The one in amdgpu is a bit 
>>>> more complex because of the added complexity for more fence timelines.
>>>>
>>>> We could take the one from amdgpu and use it for radeon and others 
>>>> as well, but the allocator proposed here doesn't even remotely 
>>>> matches the requirements.
>>>
>>> But again, what *are* those missing requirements exactly? What is 
>>> the pathological case you see for the current code?
>>
>> Well very low CPU overhead and don't do anything in a callback.
>
> Well, dma_fence_wait_any() will IIRC register callbacks on all 
> affected fences, although admittedly there is no actual allocator 
> processing in them.
>
>>
>>>
>>> From what I can tell the amdgpu suballocator introduces excessive 
>>> complexity to coalesce waits for fences from the same contexts, 
>>> whereas the present code just frees from the fence callback if the 
>>> fence wasn't already signaled.
>>
>> And this is exactly the design we had previously which we removed 
>> after Dave stumbled over tons of problems with it.
>
> So is the worry that those problems have spilled over in this code 
> then? It's been pretty extensively tested, or is it you should never 
> really use dma-fence callbacks?
>
>>
>>> The fence signalling code that fires that callback is typcally 
>>> always run anyway on scheduler fences.
>>>
>>> The reason we had for not using the amdgpu suballocator as 
>>> originally planned was that this complexity made it very hard for us 
>>> to undertand it and to fix issues we had with it.
>>
>> Well what are those problems? The idea is actually not that hardware 
>> to understand.
>
> We hit memory corruption, and we spent substantially more time trying 
> to debug it than to put together this patch, while never really 
> understanding what  happened, nor why you don't see that with amdgpu.
>
>>
>> We could simplify it massively for the cost of only waiting for the 
>> oldest fence if that helps.
>
> Let me grab the latest version from amdgpu and give it a try again, 
> but yes I think that to make it common code we'll need it simpler (and 
> my personal wish would be to separate the allocator functionality a 
> bit more from the fence waiting, which I guess should be OK if the 
> fence waiting is vastly simplified).
>
> /Thomas
>
>
>>
>>
>> Regards,
>> Christian.
>>
>>>
>>> Regards,
>>>
>>> Thomas
>>