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

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Date:   Thu, 9 Jun 2022 14:34:23 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     cl@...ux.com, penberg@...nel.org, rientjes@...gle.com,
        iamjoonsoo.kim@....com, akpm@...ux-foundation.org, vbabka@...e.cz,
        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 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.

Thanks.