Re: [PATCH 17/17] io_uring/alloc_cache: switch to array based caching

[Jens Axboe <axboe@kernel.dk>]

On 3/21/24 11:20 AM, Gabriel Krisman Bertazi wrote:
> Jens Axboe <axboe@kernel.dk> writes:
> 
>> On 3/21/24 9:59 AM, Gabriel Krisman Bertazi wrote:
>>> Jens Axboe <axboe@kernel.dk> writes:
>>>
>>>> Currently lists are being used to manage this, but lists isn't a very
>>>> good choice for as extracting the current entry necessitates touching
>>>> the next entry as well, to update the list head.
>>>>
>>>> Outside of that detail, games are also played with KASAN as the list
>>>> is inside the cached entry itself.
>>>>
>>>> Finally, all users of this need a struct io_cache_entry embedded in
>>>> their struct, which is union'ized with something else in there that
>>>> isn't used across the free -> realloc cycle.
>>>>
>>>> Get rid of all of that, and simply have it be an array. This will not
>>>> change the memory used, as we're just trading an 8-byte member entry
>>>> for the per-elem array size.
>>>>
>>>> This reduces the overhead of the recycled allocations, and it reduces
>>>> the code we have to support recycling.
>>>
>>> Hi Jens,
>>>
>>> I tried applying the entire to your for-6.10/io_uring branch to test it
>>> and only this last patch failed to apply. The tip of the branch I have
>>> is 22261e73e8d2 ("io_uring/alloc_cache: shrink default max entries from
>>> 512 to 128").
>>
>> Yeah it has some dependencies that need unraveling. The easiest is if
>> you just pull:
>>
>> git://git.kernel.dk/linux io_uring-recvsend-bundle
>>
>> into current -git master, and then just test that. That gets you pretty
>> much everything that's being tested and played with.
>>
>> Top of tree is d5653d2fcf1383c0fbe8b64545664aea36c7aca2 right now.
> 
> thanks, I'll test with that.

Thanks!

>>>> -static inline struct io_cache_entry *io_alloc_cache_get(struct io_alloc_cache *cache)
>>>> +static inline void *io_alloc_cache_get(struct io_alloc_cache *cache)
>>>>  {
>>>> -	if (cache->list.next) {
>>>> -		struct io_cache_entry *entry;
>>>> +	if (cache->nr_cached) {
>>>> +		void *entry = cache->entries[--cache->nr_cached];
>>>>  
>>>> -		entry = container_of(cache->list.next, struct io_cache_entry, node);
>>>>  		kasan_mempool_unpoison_object(entry, cache->elem_size);
>>>> -		cache->list.next = cache->list.next->next;
>>>> -		cache->nr_cached--;
>>>>  		return entry;
>>>>  	}
>>>>  
>>>>  	return NULL;
>>>>  }
>>>>  
>>>> -static inline void io_alloc_cache_init(struct io_alloc_cache *cache,
>>>> -				       unsigned max_nr, size_t size)
>>>> +static inline int io_alloc_cache_init(struct io_alloc_cache *cache,
>>>> +				      unsigned max_nr, size_t size)
>>>>  {
>>>> -	cache->list.next = NULL;
>>>> +	cache->entries = kvmalloc_array(max_nr, sizeof(void *), GFP_KERNEL);
>>>> +	if (!cache->entries)
>>>> +		return -ENOMEM;
>>>>  	cache->nr_cached = 0;
>>>>  	cache->max_cached = max_nr;
>>>>  	cache->elem_size = size;
>>>> +	return 0;
>>>>  }
>>>>  
>>>>  static inline void io_alloc_cache_free(struct io_alloc_cache *cache,
>>>> -					void (*free)(struct io_cache_entry *))
>>>> +				       void (*free)(const void *))
>>>
>>> Minor, but since free is supposed to free the entry, const doesn't
>>> make sense here.  Also, you actually just cast it away immediately in
>>> every usage.
>>
>> It's because then I can use kfree() directly for most cases, only two of
>> them have special freeing functions. And kfree takes a const void *. I
>> should add a comment about that.
> 
> TIL. For the record, I was very puzzled on why kfree receives a const
> pointer just to cast it away immediately too. Then I found Linus
> discussing it at https://yarchive.net/comp/const.html
> 
> Anyway, in this case, we are actually modifying it in io_rw_cache_free,
> and we don't need to explicitly cast from non-const to const , so I still
> think you can avoid the comment and drop the const.  But that is just
> a nitpick that i won't insist.

Right, but then I need a wrapper ala:

io_uring_kfree(void *entry)
{
	kfree(entry);
}

which obviously isn't the end of the world, but the cast is just fine
as-is imho.

-- 
Jens Axboe