Re: [PATCH v6 3/3] drm/buddy: Add defragmentation support

[Matthew Auld <matthew.auld@intel.com>]

On 16/02/2024 12:33, Christian König wrote:
> Am 16.02.24 um 13:23 schrieb Matthew Auld:
>> On 08/02/2024 15:50, Arunpravin Paneer Selvam wrote:
>>> Add a function to support defragmentation.
>>>
>>> v1: Defragment the memory beginning from min_order
>>>      till the required memory space is available.
>>>
>>> Signed-off-by: Arunpravin Paneer Selvam 
>>> <Arunpravin.PaneerSelvam@amd.com>
>>> Suggested-by: Matthew Auld <matthew.auld@intel.com>
>>> ---
>>>   drivers/gpu/drm/drm_buddy.c | 67 +++++++++++++++++++++++++++++++------
>>>   include/drm/drm_buddy.h     |  3 ++
>>
>> No users?
> 
> Other question is how can a buddy allocator fragment in the first place?

The fragmentation is due to pages now being tracked as dirty/clear. 
Should the allocator merge together a page that is dirty with a page 
that is cleared? When should it do that? User wants to be able to keep 
the two separate if possible. For example, freeing one single dirty page 
can dirty a huge swathe of your already cleared pages if they are merged 
together. Or do you have some some other ideas here?

> 
> Christian.
> 
>>
>>>   2 files changed, 59 insertions(+), 11 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/drm_buddy.c b/drivers/gpu/drm/drm_buddy.c
>>> index 33ad0cfbd54c..fac423d2cb73 100644
>>> --- a/drivers/gpu/drm/drm_buddy.c
>>> +++ b/drivers/gpu/drm/drm_buddy.c
>>> @@ -276,10 +276,12 @@ drm_get_buddy(struct drm_buddy_block *block)
>>>   }
>>>   EXPORT_SYMBOL(drm_get_buddy);
>>>   -static void __drm_buddy_free(struct drm_buddy *mm,
>>> -                 struct drm_buddy_block *block)
>>> +static unsigned int __drm_buddy_free(struct drm_buddy *mm,
>>> +                     struct drm_buddy_block *block,
>>> +                     bool defrag)
>>>   {
>>>       struct drm_buddy_block *parent;
>>> +    unsigned int order;
>>>         while ((parent = block->parent)) {
>>>           struct drm_buddy_block *buddy;
>>> @@ -289,12 +291,14 @@ static void __drm_buddy_free(struct drm_buddy *mm,
>>>           if (!drm_buddy_block_is_free(buddy))
>>>               break;
>>>   -        if (drm_buddy_block_is_clear(block) !=
>>> -            drm_buddy_block_is_clear(buddy))
>>> -            break;
>>> +        if (!defrag) {
>>> +            if (drm_buddy_block_is_clear(block) !=
>>> +                drm_buddy_block_is_clear(buddy))
>>> +                break;
>>>   -        if (drm_buddy_block_is_clear(block))
>>> -            mark_cleared(parent);
>>> +            if (drm_buddy_block_is_clear(block))
>>> +                mark_cleared(parent);
>>> +        }
>>
>> Maybe check if the two blocks are incompatible and chuck a warn if 
>> they are not? Main thing is not to hide issues with split blocks that 
>> should have been merged before.
>>
>>>             list_del(&buddy->link);
>>>   @@ -304,8 +308,49 @@ static void __drm_buddy_free(struct drm_buddy 
>>> *mm,
>>>           block = parent;
>>>       }
>>>   +    order = drm_buddy_block_order(block);
>>>       mark_free(mm, block);
>>> +
>>> +    return order;
>>> +}
>>> +
>>> +/**
>>> + * drm_buddy_defrag - Defragmentation routine
>>> + *
>>> + * @mm: DRM buddy manager
>>> + * @min_order: minimum order in the freelist to begin
>>> + * the defragmentation process
>>> + *
>>> + * Driver calls the defragmentation function when the
>>> + * requested memory allocation returns -ENOSPC.
>>> + */
>>> +void drm_buddy_defrag(struct drm_buddy *mm,
>>> +              unsigned int min_order)
>>
>> Just wondering if we need "full defag" also? We would probably need to 
>> call this at fini() anyway.
>>
>>> +{
>>> +    struct drm_buddy_block *block;
>>> +    struct list_head *list;
>>> +    unsigned int order;
>>> +    int i;
>>> +
>>> +    if (min_order > mm->max_order)
>>> +        return;
>>> +
>>> +    for (i = min_order - 1; i >= 0; i--) {
>>
>> Need to be careful with min_order = 0 ?
>>
>>> +        list = &mm->free_list[i];
>>> +        if (list_empty(list))
>>> +            continue;
>>> +
>>> +        list_for_each_entry_reverse(block, list, link) {
>>
>> Don't we need the safe_reverse() variant here, since this is removing 
>> from the list?
>>
>>> +            if (!block->parent)
>>> +                continue;
>>> +
>>> +            order = __drm_buddy_free(mm, block, 1);
>>> +            if (order >= min_order)
>>> +                return;
>>> +        }
>>> +    }
>>>   }
>>> +EXPORT_SYMBOL(drm_buddy_defrag);
>>>     /**
>>>    * drm_buddy_free_block - free a block
>>> @@ -321,7 +366,7 @@ void drm_buddy_free_block(struct drm_buddy *mm,
>>>       if (drm_buddy_block_is_clear(block))
>>>           mm->clear_avail += drm_buddy_block_size(mm, block);
>>>   -    __drm_buddy_free(mm, block);
>>> +    __drm_buddy_free(mm, block, 0);
>>>   }
>>>   EXPORT_SYMBOL(drm_buddy_free_block);
>>>   @@ -470,7 +515,7 @@ __alloc_range_bias(struct drm_buddy *mm,
>>>       if (buddy &&
>>>           (drm_buddy_block_is_free(block) &&
>>>            drm_buddy_block_is_free(buddy)))
>>> -        __drm_buddy_free(mm, block);
>>> +        __drm_buddy_free(mm, block, 0);
>>>       return ERR_PTR(err);
>>>   }
>>>   @@ -588,7 +633,7 @@ alloc_from_freelist(struct drm_buddy *mm,
>>>     err_undo:
>>>       if (tmp != order)
>>> -        __drm_buddy_free(mm, block);
>>> +        __drm_buddy_free(mm, block, 0);
>>>       return ERR_PTR(err);
>>>   }
>>>   @@ -668,7 +713,7 @@ static int __alloc_range(struct drm_buddy *mm,
>>>       if (buddy &&
>>>           (drm_buddy_block_is_free(block) &&
>>>            drm_buddy_block_is_free(buddy)))
>>> -        __drm_buddy_free(mm, block);
>>> +        __drm_buddy_free(mm, block, 0);
>>>     err_free:
>>>       if (err == -ENOSPC && total_allocated_on_err) {
>>> diff --git a/include/drm/drm_buddy.h b/include/drm/drm_buddy.h
>>> index d81c596dfa38..d0f63e7b5915 100644
>>> --- a/include/drm/drm_buddy.h
>>> +++ b/include/drm/drm_buddy.h
>>> @@ -166,6 +166,9 @@ void drm_buddy_free_list(struct drm_buddy *mm,
>>>                struct list_head *objects,
>>>                unsigned int flags);
>>>   +void drm_buddy_defrag(struct drm_buddy *mm,
>>> +              unsigned int min_order);
>>> +
>>>   void drm_buddy_print(struct drm_buddy *mm, struct drm_printer *p);
>>>   void drm_buddy_block_print(struct drm_buddy *mm,
>>>                  struct drm_buddy_block *block,
>