Re: [Intel-gfx] [PATCH v4 4/6] drm: implement a method to free unused pages

[Arunpravin <arunpravin.paneerselvam@amd.com>]

On 14/12/21 12:10 am, Matthew Auld wrote:
> On 01/12/2021 16:39, Arunpravin wrote:
>> On contiguous allocation, we round up the size
>> to the *next* power of 2, implement a function
>> to free the unused pages after the newly allocate block.
>>
>> v2(Matthew Auld):
>>    - replace function name 'drm_buddy_free_unused_pages' with
>>      drm_buddy_block_trim
>>    - replace input argument name 'actual_size' with 'new_size'
>>    - add more validation checks for input arguments
>>    - add overlaps check to avoid needless searching and splitting
>>    - merged the below patch to see the feature in action
>>      - add free unused pages support to i915 driver
>>    - lock drm_buddy_block_trim() function as it calls mark_free/mark_split
>>      are all globally visible
>>
>> v3:
>>    - remove drm_buddy_block_trim() error handling and
>>      print a warn message if it fails
>>
>> Signed-off-by: Arunpravin <Arunpravin.PaneerSelvam@amd.com>
>> ---
>>   drivers/gpu/drm/drm_buddy.c                   | 72 ++++++++++++++++++-
>>   drivers/gpu/drm/i915/i915_ttm_buddy_manager.c | 10 +++
>>   include/drm/drm_buddy.h                       |  4 ++
>>   3 files changed, 83 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/drm_buddy.c b/drivers/gpu/drm/drm_buddy.c
>> index eddc1eeda02e..707efc82216d 100644
>> --- a/drivers/gpu/drm/drm_buddy.c
>> +++ b/drivers/gpu/drm/drm_buddy.c
>> @@ -434,7 +434,8 @@ alloc_from_freelist(struct drm_buddy_mm *mm,
>>   static int __alloc_range(struct drm_buddy_mm *mm,
>>   			 struct list_head *dfs,
>>   			 u64 start, u64 size,
>> -			 struct list_head *blocks)
>> +			 struct list_head *blocks,
>> +			 bool trim_path)
>>   {
>>   	struct drm_buddy_block *block;
>>   	struct drm_buddy_block *buddy;
>> @@ -480,8 +481,20 @@ static int __alloc_range(struct drm_buddy_mm *mm,
>>   
>>   		if (!drm_buddy_block_is_split(block)) {
>>   			err = split_block(mm, block);
>> -			if (unlikely(err))
>> +			if (unlikely(err)) {
>> +				if (trim_path)
>> +					/*
>> +					 * Here in case of trim, we return and dont goto
>> +					 * split failure path as it removes from the
>> +					 * original list and potentially also freeing
>> +					 * the block. so we could leave as it is,
>> +					 * worse case we get some internal fragmentation
>> +					 * and leave the decision to the user
>> +					 */
>> +					return err;
> 
> Hmm, ideally we don't want to leave around blocks where both buddies are 
> free without then also merging them back(not sure if that trips some 
> BUG_ON). Also IIUC, if we hit this failure path, depending on where the 
> split_block() fails we might be allocating something less than new_size? 
> Also if it's the first split_block() that fails then the user just gets 
> an empty list?
> 
> Could we perhaps just turn this node into a temporary root node to 
> prevent recursively freeing itself, but still retain the 
> un-splitting/freeing of the other nodes i.e something like:
> 
> list_del(&block->link);
> mark_free(mm, block);
> mm->avail += ...;
> 
> /* Prevent recursively freeing this node */
> parent = block->parent;
> block->parent = NULL;
> 
> list_add(&block->tmp_link, &dfs);
> ret = _alloc_range(mm, &dfs, new_start, new_size, blocks);
> if (ret) {
>      mem->avail -= ...;
>      mark_allocated(block);
>      list_add(&block->link, blocks);
> }
> 
> block->parent = parent;
> return ret;
> 
> That way we can also drop the special trim_path handling. Thoughts?

That's a nice idea. It will work.
> 
>> +
>>   				goto err_undo;
>> +			}
>>   		}
>>   
>>   		list_add(&block->right->tmp_link, dfs);
>> @@ -535,8 +548,61 @@ static int __drm_buddy_alloc_range(struct drm_buddy_mm *mm,
>>   	for (i = 0; i < mm->n_roots; ++i)
>>   		list_add_tail(&mm->roots[i]->tmp_link, &dfs);
>>   
>> -	return __alloc_range(mm, &dfs, start, size, blocks);
>> +	return __alloc_range(mm, &dfs, start, size, blocks, 0);
>> +}
>> +
>> +/**
>> + * drm_buddy_block_trim - free unused pages
>> + *
>> + * @mm: DRM buddy manager
>> + * @new_size: original size requested
>> + * @blocks: output list head to add allocated blocks
>> + *
>> + * For contiguous allocation, we round up the size to the nearest
>> + * power of two value, drivers consume *actual* size, so remaining
>> + * portions are unused and it can be freed.
>> + *
>> + * Returns:
>> + * 0 on success, error code on failure.
>> + */
>> +int drm_buddy_block_trim(struct drm_buddy_mm *mm,
>> +			 u64 new_size,
>> +			 struct list_head *blocks)
>> +{
>> +	struct drm_buddy_block *block;
>> +	u64 new_start;
>> +	LIST_HEAD(dfs);
>> +
>> +	if (!list_is_singular(blocks))
>> +		return -EINVAL;
>> +
>> +	block = list_first_entry(blocks,
>> +				 struct drm_buddy_block,
>> +				 link);
>> +
>> +	if (!drm_buddy_block_is_allocated(block))
>> +		return -EINVAL;
>> +
>> +	if (new_size > drm_buddy_block_size(mm, block))
>> +		return -EINVAL;
>> +
>> +	if (!new_size && !IS_ALIGNED(new_size, mm->chunk_size))
>> +		return -EINVAL;
>> +
>> +	if (new_size == drm_buddy_block_size(mm, block))
>> +		return 0;
>> +
>> +	list_del(&block->link);
>> +
>> +	new_start = drm_buddy_block_offset(block);
>> +
>> +	mark_free(mm, block);
>> +
>> +	list_add(&block->tmp_link, &dfs);
>> +
>> +	return __alloc_range(mm, &dfs, new_start, new_size, blocks, 1);
>>   }
>> +EXPORT_SYMBOL(drm_buddy_block_trim);
>>   
>>   /**
>>    * drm_buddy_alloc - allocate power-of-two blocks
>> diff --git a/drivers/gpu/drm/i915/i915_ttm_buddy_manager.c b/drivers/gpu/drm/i915/i915_ttm_buddy_manager.c
>> index 7c58efb60dba..c5831c27fe82 100644
>> --- a/drivers/gpu/drm/i915/i915_ttm_buddy_manager.c
>> +++ b/drivers/gpu/drm/i915/i915_ttm_buddy_manager.c
>> @@ -97,6 +97,16 @@ static int i915_ttm_buddy_man_alloc(struct ttm_resource_manager *man,
>>   	if (unlikely(err))
>>   		goto err_free_blocks;
>>   
>> +	if (place->flags & TTM_PL_FLAG_CONTIGUOUS) {
>> +		mutex_lock(&bman->lock);
>> +		err = drm_buddy_block_trim(mm,
>> +				(u64)n_pages << PAGE_SHIFT,
>> +				&bman_res->blocks);
>> +		mutex_unlock(&bman->lock);
>> +		pr_warn("drm_buddy_block_trim failed returing %d for ttm_buffer_object(%p)\n",
>> +			err, bo);
> 
> 
> IIUC this prints a warning even for success? Anyway, I think we can drop 
> the pr_warn, since normal user can't really do much here?
Yes, I will drop pr_warn
> 
>> +	}
>> +
>>   	*res = &bman_res->base;
>>   	return 0;
>>   
>> diff --git a/include/drm/drm_buddy.h b/include/drm/drm_buddy.h
>> index 316ac0d25f08..90906d9dbbf0 100644
>> --- a/include/drm/drm_buddy.h
>> +++ b/include/drm/drm_buddy.h
>> @@ -146,6 +146,10 @@ int drm_buddy_alloc(struct drm_buddy_mm *mm,
>>   		    struct list_head *blocks,
>>   		    unsigned long flags);
>>   
>> +int drm_buddy_block_trim(struct drm_buddy_mm *mm,
>> +			 u64 new_size,
>> +			 struct list_head *blocks);
>> +
>>   void drm_buddy_free(struct drm_buddy_mm *mm, struct drm_buddy_block *block);
>>   
>>   void drm_buddy_free_list(struct drm_buddy_mm *mm, struct list_head *objects);
>>