Re: [PATCH RFC 1/4] mm, slab: move memcg charging to post-alloc hook

Vlastimil Babka
2024-03-13T10:55:03Z

On 3/12/24 19:52, Roman Gushchin wrote:
> On Fri, Mar 01, 2024 at 06:07:08PM +0100, Vlastimil Babka wrote:
>> The MEMCG_KMEM integration with slab currently relies on two hooks
>> during allocation. memcg_slab_pre_alloc_hook() determines the objcg and
>> charges it, and memcg_slab_post_alloc_hook() assigns the objcg pointer
>> to the allocated object(s).
>>
>> As Linus pointed out, this is unnecessarily complex. Failing to charge
>> due to memcg limits should be rare, so we can optimistically allocate
>> the object(s) and do the charging together with assigning the objcg
>> pointer in a single post_alloc hook. In the rare case the charging
>> fails, we can free the object(s) back.
>>
>> This simplifies the code (no need to pass around the objcg pointer) and
>> potentially allows to separate charging from allocation in cases where
>> it's common that the allocation would be immediately freed, and the
>> memcg handling overhead could be saved.
>>
>> Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
>> Link: https://lore.kernel.org/all/CAHk-=whYOOdM7jWy5jdrAm8LxcgCMFyk2bt8fYYvZzM4U-zAQA@mail.gmail.com/
>> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
>
> Nice cleanup, Vlastimil!
> Couple of small nits below, but otherwise, please, add my
>
> Reviewed-by: Roman Gushchin <roman.gushchin@linux.dev>

Thanks!

>> /*
>> * The obtained objcg pointer is safe to use within the current scope,
>> * defined by current task or set_active_memcg() pair.
>> * obj_cgroup_get() is used to get a permanent reference.
>> */
>> - struct obj_cgroup *objcg = current_obj_cgroup();
>> + objcg = current_obj_cgroup();
>> if (!objcg)
>> return true;
>>
>> + /*
>> + * slab_alloc_node() avoids the NULL check, so we might be called with a
>> + * single NULL object. kmem_cache_alloc_bulk() aborts if it can't fill
>> + * the whole requested size.
>> + * return success as there's nothing to free back
>> + */
>> + if (unlikely(*p == NULL))
>> + return true;
>
> Probably better to move this check up? current_obj_cgroup() != NULL check is more
> expensive.

It probably doesn't matter in practice anyway, but my thinking was that
*p == NULL is so rare (the object allocation failed) it shouldn't matter
that we did current_obj_cgroup() uselessly in case it happens.
OTOH current_obj_cgroup() returning NULL is not that rare (?) so it
could be useful to not check *p in those cases?

>> +
>> + flags &= gfp_allowed_mask;
>> +
>> if (lru) {
>> int ret;
>> struct mem_cgroup *memcg;
>> @@ -1926,71 +1939,51 @@ static bool __memcg_slab_pre_alloc_hook(struct kmem_cache *s,
>> return false;
>> }
>>
>> - if (obj_cgroup_charge(objcg, flags, objects * obj_full_size(s)))
>> + if (obj_cgroup_charge(objcg, flags, size * obj_full_size(s)))
>> return false;
>>
>> - *objcgp = objcg;
>> + for (i = 0; i < size; i++) {
>> + slab = virt_to_slab(p[i]);
>
> Not specific to this change, but I wonder if it makes sense to introduce virt_to_slab()
> variant without any extra checks for this and similar cases, where we know for sure
> that p resides on a slab page. What do you think?