Re: [PATCH v2 1/2] mm, slab: move memcg charging to post-alloc hook

[Aishwarya TCV <aishwarya.tcv@arm.com>]

On 03/04/2024 16:48, Vlastimil Babka wrote:
> On 4/3/24 1:39 PM, Aishwarya TCV wrote:
>>
>>
>> On 25/03/2024 08:20, 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/
>>> Reviewed-by: Roman Gushchin <roman.gushchin@linux.dev>
>>> Reviewed-by: Chengming Zhou <chengming.zhou@linux.dev>
>>> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
>>> ---
>>>  mm/slub.c | 180 +++++++++++++++++++++++++++-----------------------------------
>>>  1 file changed, 77 insertions(+), 103 deletions(-)
>>
>> Hi Vlastimil,
>>
>> When running the LTP test "memcg_limit_in_bytes" against next-master
>> (next-20240402) kernel with Arm64 on JUNO, oops is observed in our CI. I
>> can send the full logs if required. It is observed to work fine on
>> softiron-overdrive-3000.
>>
>> A bisect identified 11bb2d9d91627935c63ea3e6a031fd238c846e1 as the first
>> bad commit. Bisected it on the tag "next-20240402" at repo
>> "https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git".
>>
>> This works fine on  Linux version v6.9-rc2
> 
> Oops, sorry, can you verify that this fixes it?
> Thanks.
> 
> ----8<----
> From b0597c220624fef4f10e26079a3ff1c86f02a12b Mon Sep 17 00:00:00 2001
> From: Vlastimil Babka <vbabka@suse.cz>
> Date: Wed, 3 Apr 2024 17:45:15 +0200
> Subject: [PATCH] fixup! mm, slab: move memcg charging to post-alloc hook
> 
> The call to memcg_alloc_abort_single() is wrong, it expects a pointer to
> single object, not an array.
> 
> Reported-by: Aishwarya TCV <aishwarya.tcv@arm.com>
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> ---
>  mm/slub.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index f5b151a58b7d..b32e79629ae7 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -2100,7 +2100,7 @@ bool memcg_slab_post_alloc_hook(struct kmem_cache *s, struct list_lru *lru,
>  		return true;
>  
>  	if (likely(size == 1)) {
> -		memcg_alloc_abort_single(s, p);
> +		memcg_alloc_abort_single(s, *p);
>  		*p = NULL;
>  	} else {
>  	
	kmem_cache_free_bulk(s, size, p);

Tested the attached patch on next-20240302. Confirming that the test is
running fine. Test run log is attached below.

Test run log:
--------------
memcg_limit_in_bytes 8 TPASS: process 614 is killed
memcg_limit_in_bytes 9 TINFO: Test limit_in_bytes will be aligned to
PAGESIZE
memcg_limit_in_bytes 9 TPASS: echo 4095 > memory.limit_in_bytes passed
as expected
memcg_limit_in_bytes 9 TPASS: input=4095, limit_in_bytes=0
memcg_limit_in_bytes 10 TPASS: echo 4097 > memory.limit_in_bytes passed
as expected
memcg_limit_in_bytes 10 TPASS: input=4097, limit_in_bytes=4096
memcg_limit_in_bytes 11 TPASS: echo 1 > memory.limit_in_bytes passed as
expected
memcg_limit_in_bytes 11 TPASS: input=1, limit_in_bytes=0
memcg_limit_in_bytes 12 TINFO: Test invalid memory.limit_in_bytes
memcg_limit_in_bytes 12 TPASS: echo -1 > memory.limit_in_bytes passed as
expected
memcg_limit_in_bytes 13 TPASS: echo 1.0 > memory.limit_in_bytes failed
as expected
memcg_limit_in_bytes 14 TPASS: echo 1xx > memory.limit_in_bytes failed
as expected
memcg_limit_in_bytes 15 TPASS: echo xx > memory.limit_in_bytes failed as
expected
Summary:
passed   18
failed   0
broken   0
skipped  0
warnings 0

Thanks,
Aishwarya