linux-kernel - Re: [RFC PATCH v3 6/7] slub: Delay freezing of partial slabs

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Date:   Tue, 31 Oct 2023 10:50:17 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     chengming.zhou@...ux.dev, cl@...ux.com, penberg@...nel.org
Cc:     rientjes@...gle.com, iamjoonsoo.kim@....com,
        akpm@...ux-foundation.org, roman.gushchin@...ux.dev,
        42.hyeyoo@...il.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Chengming Zhou <zhouchengming@...edance.com>
Subject: Re: [RFC PATCH v3 6/7] slub: Delay freezing of partial slabs

On 10/24/23 11:33, chengming.zhou@...ux.dev wrote:
> From: Chengming Zhou <zhouchengming@...edance.com>
> 
> Now we will freeze slabs when moving them out of node partial list to
> cpu partial list, this method needs two cmpxchg_double operations:
> 
> 1. freeze slab (acquire_slab()) under the node list_lock
> 2. get_freelist() when pick used in ___slab_alloc()
> 
> Actually we don't need to freeze when moving slabs out of node partial
> list, we can delay freezing to when use slab freelist in ___slab_alloc(),
> so we can save one cmpxchg_double().
> 
> And there are other good points:
>  - The moving of slabs between node partial list and cpu partial list
>    becomes simpler, since we don't need to freeze or unfreeze at all.
> 
>  - The node list_lock contention would be less, since we don't need to
>    freeze any slab under the node list_lock.
> 
> We can achieve this because there is no concurrent path would manipulate
> the partial slab list except the __slab_free() path, which is serialized
> now.

"which is now serialized by slab_test_node_partial() under the list_lock." ?

> Since the slab returned by get_partial() interfaces is not frozen anymore
> and no freelist in the partial_context, so we need to use the introduced

		 ^ is returned in

> freeze_slab() to freeze it and get its freelist.
> 
> Similarly, the slabs on the CPU partial list are not frozen anymore,
> we need to freeze_slab() on it before use.

We can now delete acquire_slab() as it became unused.

> Signed-off-by: Chengming Zhou <zhouchengming@...edance.com>

With the fixup for CONFIG_SLUB_CPU_PARTIAL you mentioned,
Reviewed-by: Vlastimil Babka <vbabka@...e.cz>

Also agreed with followup patch to rename unfreeze_partials().
Thanks!