Re: [PATCH] drm/amdgpu: restructure amdgpu_vram_mgr_new

[Christian König <ckoenig.leichtzumerken@xxxxxxxxx>]

Am 26.04.21 um 18:02 schrieb Felix Kuehling:
> Am 2021-04-26 um 4:54 a.m. schrieb Christian König:
> > Merge the two loops, loosen the restriction for big allocations.
> > This reduces the CPU overhead in the good case, but increases
> > it a bit under memory pressure.
> >
> > Signed-off-by: Christian König <christian.koenig@xxxxxxx>
> > ---
> >   drivers/gpu/drm/amd/amdgpu/amdgpu_vram_mgr.c | 58 +++++++++-----------
> >   1 file changed, 27 insertions(+), 31 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_vram_mgr.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_vram_mgr.c
> > index 529c5c32a205..e2cbe19404c0 100644
> > --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_vram_mgr.c
> > +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_vram_mgr.c
> > @@ -358,13 +358,13 @@ static int amdgpu_vram_mgr_new(struct ttm_resource_manager *man,
> >   			       const struct ttm_place *place,
> >   			       struct ttm_resource *mem)
> >   {
> > +	unsigned long lpfn, num_nodes, pages_per_node, pages_left, pages;
> >   	struct amdgpu_vram_mgr *mgr = to_vram_mgr(man);
> >   	struct amdgpu_device *adev = to_amdgpu_device(mgr);
> > +	uint64_t vis_usage = 0, mem_bytes, max_bytes;
> >   	struct drm_mm *mm = &mgr->mm;
> > -	struct drm_mm_node *nodes;
> >   	enum drm_mm_insert_mode mode;
> > -	unsigned long lpfn, num_nodes, pages_per_node, pages_left;
> > -	uint64_t vis_usage = 0, mem_bytes, max_bytes;
> > +	struct drm_mm_node *nodes;
> >   	unsigned i;
> >   	int r;
> >   
> > @@ -391,9 +391,10 @@ static int amdgpu_vram_mgr_new(struct ttm_resource_manager *man,
> >   		pages_per_node = HPAGE_PMD_NR;
> >   #else
> >   		/* default to 2MB */
> > -		pages_per_node = (2UL << (20UL - PAGE_SHIFT));
> > +		pages_per_node = 2UL << (20UL - PAGE_SHIFT);
> >   #endif
> > -		pages_per_node = max((uint32_t)pages_per_node, mem->page_alignment);
> > +		pages_per_node = max_t(uint32_t, pages_per_node,
> > +				       mem->page_alignment);
> >   		num_nodes = DIV_ROUND_UP(mem->num_pages, pages_per_node);
> >   	}
> >   
> > @@ -411,42 +412,37 @@ static int amdgpu_vram_mgr_new(struct ttm_resource_manager *man,
> >   	mem->start = 0;
> >   	pages_left = mem->num_pages;
> >   
> > -	spin_lock(&mgr->lock);
> > -	for (i = 0; pages_left >= pages_per_node; ++i) {
> > -		unsigned long pages = rounddown_pow_of_two(pages_left);
> > -
> > -		/* Limit maximum size to 2GB due to SG table limitations */
> > -		pages = min(pages, (2UL << (30 - PAGE_SHIFT)));
> > +	/* Limit maximum size to 2GB due to SG table limitations */
> > +	pages = min(pages_left, 2UL << (30 - PAGE_SHIFT));
> >   
> > -		r = drm_mm_insert_node_in_range(mm, &nodes[i], pages,
> > -						pages_per_node, 0,
> > -						place->fpfn, lpfn,
> > -						mode);
> > -		if (unlikely(r))
> > -			break;
> > -
> > -		vis_usage += amdgpu_vram_mgr_vis_size(adev, &nodes[i]);
> > -		amdgpu_vram_mgr_virt_start(mem, &nodes[i]);
> > -		pages_left -= pages;
> > -	}
> > -
> > -	for (; pages_left; ++i) {
> > -		unsigned long pages = min(pages_left, pages_per_node);
> > +	i = 0;
> > +	spin_lock(&mgr->lock);
> > +	while (pages_left) {
> >   		uint32_t alignment = mem->page_alignment;
> >   
> > -		if (pages == pages_per_node)
> > +		if (pages >= pages_per_node)
> >   			alignment = pages_per_node;
> >   
> > -		r = drm_mm_insert_node_in_range(mm, &nodes[i],
> > -						pages, alignment, 0,
> > -						place->fpfn, lpfn,
> > -						mode);
> > -		if (unlikely(r))
> > +		r = drm_mm_insert_node_in_range(mm, &nodes[i], pages, alignment,
> > +						0, place->fpfn, lpfn, mode);
> > +		if (unlikely(r)) {
> > +			if (pages > pages_per_node) {
> This means we can never allocate chunks smaller than 2MB, except for the
> tail. And the tail still needs to be allocated in one piece if it's < 2MB.

Correct, but that was the behavior before as well.

> On the other hand, we should not allow allocations smaller than
> mem->page_alignment, except for the tail. So should this condition be
> "if (pages > mem->page_alignment)" to allow maximum flexibility for
> allocations without physical alignment constraints when memory is very
> fragmented?

See a few lines above:

pages_per_node = max_t(uint32_t, pages_per_node, page_alignment);

So pages_per_node is always larger than page_alignment and we actually 
can't allocate less than pages_per_node in one allocation or we would 
overflow the nodes array.

Regards,
Christian.

> Regards,
>    Felix
>
>
> > +				if (is_power_of_2(pages))
> > +					pages = pages / 2;
> > +				else
> > +					pages = rounddown_pow_of_two(pages);
> > +				continue;
> > +			}
> >   			goto error;
> > +		}
> >   
> >   		vis_usage += amdgpu_vram_mgr_vis_size(adev, &nodes[i]);
> >   		amdgpu_vram_mgr_virt_start(mem, &nodes[i]);
> >   		pages_left -= pages;
> > +		++i;
> > +
> > +		if (pages > pages_left)
> > +			pages = pages_left;
> >   	}
> >   	spin_unlock(&mgr->lock);
> >   

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