linux-kernel - Re: [PATCH v2] mm: slab: optimize memcg_slab_free

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Date:   Tue, 14 Jun 2022 14:59:02 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Muchun Song <songmuchun@...edance.com>, cl@...ux.com,
        penberg@...nel.org, rientjes@...gle.com, iamjoonsoo.kim@....com,
        akpm@...ux-foundation.org, roman.gushchin@...ux.dev
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org, smuchun@...il.com
Subject: Re: [PATCH v2] mm: slab: optimize memcg_slab_free_hook()

On 6/9/22 08:34, Muchun Song wrote:
> On Fri, Apr 29, 2022 at 08:30:44PM +0800, Muchun Song wrote:
>> Most callers of memcg_slab_free_hook() already know the slab,  which could
>> be passed to memcg_slab_free_hook() directly to reduce the overhead of an
>> another call of virt_to_slab().  For bulk freeing of objects, the call of
>> slab_objcgs() in the loop in memcg_slab_free_hook() is redundant as well.
>> Rework memcg_slab_free_hook() and build_detached_freelist() to reduce
>> those unnecessary overhead and make memcg_slab_free_hook() can handle bulk
>> freeing in slab_free().
>> 
>> Move the calling site of memcg_slab_free_hook() from do_slab_free() to
>> slab_free() for slub to make the code clearer since the logic is weird
>> (e.g. the caller need to judge whether it needs to call
>> memcg_slab_free_hook()). It is easy to make mistakes like missing calling
>> of memcg_slab_free_hook() like fixes of:
>> 
>>   commit d1b2cf6cb84a ("mm: memcg/slab: uncharge during kmem_cache_free_bulk()")
>>   commit ae085d7f9365 ("mm: kfence: fix missing objcg housekeeping for SLAB")
>> 
>> This optimization is mainly for bulk objects freeing.  The following numbers
>> is shown for 16-object freeing.
>> 
>>                            before      after
>>   kmem_cache_free_bulk:   ~430 ns     ~400 ns
>> 
>> The overhead is reduced by about 7% for 16-object freeing.
>> 
>> Signed-off-by: Muchun Song <songmuchun@...edance.com>
> 
> Hi Vlastimil,
> 
> Wolud you mind picking it up? I did not see this patch on the
> slab tree.

Sorry, was waiting for rc1 to start the for-5.20 branches and was away for
another week then. Now pushed to slab/for-5.20/optimizations

> Thanks.
>