[RFC 1/5] drm/amdgpu: Fix migration rate limiting accounting

[Friedrich Vock]

On 09.05.24 11:19, Tvrtko Ursulin wrote:
>
> On 08/05/2024 20:08, Friedrich Vock wrote:
>> On 08.05.24 20:09, Tvrtko Ursulin wrote:
>>> From: Tvrtko Ursulin <tvrtko.ursulin at igalia.com>
>>>
>>> The logic assumed any migration attempt worked and therefore would
>>> over-
>>> account the amount of data migrated during buffer re-validation. As a
>>> consequence client can be unfairly penalised by incorrectly considering
>>> its migration budget spent.
>>
>> If the migration failed but data was still moved (which I think could be
>> the case when we try evicting everything but it still doesn't work?),
>> shouldn't the eviction movements count towards the ratelimit too?
>
> Possibly, which path would that be?
>
Thinking about it more, the only case where allocation still won't
succeed after evicting everything from a place is the edge case when the
buffer is larger than the place's size.

The most likely condition for this to happen (without the submission
failing entirely because the buffer just doesn't fit anywhere) would be
if the app tries creating a 256MB+ visible-VRAM buffer if resizeable BAR
is disabled.
This case could potentially trigger an allocation failure when trying
with preferred_domains, but retrying with allowed_domains, which
includes GTT, could subsequently work.

> I mean there are definitely more migration which *should not* be
> counted which I think your mini-series approaches more accurately.
> What this patch achieves, in its current RFC form, is reduces the
> "false-positive" migration budget depletions.
>
> So larger improvements aside, point of the series was to illustrate
> that even the things which were said to be working do not seem to. See
> cover letter to see what I thought does not work either well or at all.
Fair point. If this patchset does "wrong"/inaccurate accounting in a
different way that improves the experience, then it's still an improvement.
>>> Fix it by looking at the before and after buffer object backing
>>> store and
>>> only account if there was a change.
>>>
>>> FIXME:
>>> I think this needs a better solution to account for migrations between
>>> VRAM visible and non-visible portions.
>>
>> FWIW, I have some WIP patches (not posted on any MLs yet though) that
>> attempt to solve this issue (+actually enforcing ratelimits) by moving
>> the ratelimit accounting/enforcement to TTM entirely.
>>
>> By moving the accounting to TTM we can count moved bytes when we move
>> them, and don't have to rely on comparing resources to determine whether
>> moving actually happened. This should address your FIXME as well.
>
> Yep, I've seen them. They are not necessarily conflicting with this
> series, potentialy TTM placement flag aside. *If* something like this
> can be kept small and still manage to fix up a few simple things which
> do not appear to work at all at the moment.
>
> For the larger re-work it is quite, well, large and it is not easy to
> be certain the end result would work as expected. IMO it would be best
> to sketch out a larger series which brings some practical and
> masurable change in behaviour before commiting to merge things piecemeal.
>
Yeah, fully agree. Getting something working and iterating on that based
on the results you get seems like the best way forward, that's what I'll
be focusing on for now.

Thanks,
Friedrich

> For instance I have a niggling feeling the runtime games driver plays
> with placements and domains are not great and wonder if things could
> be cleaner if simplified by letting TTM manage things more, more
> explicitly, and having the list of placements more static. Thinking
> about it seems a step too far for now though.
>
> Regards,
>
> Tvrtko
>
>>
>> Regards,
>> Friedrich
>>
>>> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin at igalia.com>
>>> Cc: Christian König <christian.koenig at amd.com>
>>> Cc: Friedrich Vock <friedrich.vock at gmx.de>
>>> ---
>>>   drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c | 26
>>> +++++++++++++++++++++-----
>>>   1 file changed, 21 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c
>>> b/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c
>>> index ec888fc6ead8..22708954ae68 100644
>>> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c
>>> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c
>>> @@ -784,12 +784,15 @@ static int amdgpu_cs_bo_validate(void *param,
>>> struct amdgpu_bo *bo)
>>>           .no_wait_gpu = false,
>>>           .resv = bo->tbo.base.resv
>>>       };
>>> +    struct ttm_resource *old_res;
>>>       uint32_t domain;
>>>       int r;
>>>
>>>       if (bo->tbo.pin_count)
>>>           return 0;
>>>
>>> +    old_res = bo->tbo.resource;
>>> +
>>>       /* Don't move this buffer if we have depleted our allowance
>>>        * to move it. Don't move anything if the threshold is zero.
>>>        */
>>> @@ -817,16 +820,29 @@ static int amdgpu_cs_bo_validate(void *param,
>>> struct amdgpu_bo *bo)
>>>       amdgpu_bo_placement_from_domain(bo, domain);
>>>       r = ttm_bo_validate(&bo->tbo, &bo->placement, &ctx);
>>>
>>> -    p->bytes_moved += ctx.bytes_moved;
>>> -    if (!amdgpu_gmc_vram_full_visible(&adev->gmc) &&
>>> -        amdgpu_res_cpu_visible(adev, bo->tbo.resource))
>>> -        p->bytes_moved_vis += ctx.bytes_moved;
>>> -
>>>       if (unlikely(r == -ENOMEM) && domain != bo->allowed_domains) {
>>>           domain = bo->allowed_domains;
>>>           goto retry;
>>>       }
>>>
>>> +    if (!r) {
>>> +        struct ttm_resource *new_res = bo->tbo.resource;
>>> +        bool moved = true;
>>> +
>>> +        if (old_res == new_res)
>>> +            moved = false;
>>> +        else if (old_res && new_res &&
>>> +             old_res->mem_type == new_res->mem_type)
>>> +            moved = false;
>>> +
>>> +        if (moved) {
>>> +            p->bytes_moved += ctx.bytes_moved;
>>> +            if (!amdgpu_gmc_vram_full_visible(&adev->gmc) &&
>>> +                amdgpu_res_cpu_visible(adev, bo->tbo.resource))
>>> +                p->bytes_moved_vis += ctx.bytes_moved;
>>> +        }
>>> +    }
>>> +
>>>       return r;
>>>   }
>>>