linux-kernel - Re: [PATCH v5 09/14] mm, slub: skip percpu sheaves for remote object freeing

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Message-ID: <aKvzAvgwUyZN5GC6@hyeyoo>
Date: Mon, 25 Aug 2025 14:22:10 +0900
From: Harry Yoo <harry.yoo@...cle.com>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Suren Baghdasaryan <surenb@...gle.com>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Christoph Lameter <cl@...two.org>,
        David Rientjes <rientjes@...gle.com>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Uladzislau Rezki <urezki@...il.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, rcu@...r.kernel.org,
        maple-tree@...ts.infradead.org
Subject: Re: [PATCH v5 09/14] mm, slub: skip percpu sheaves for remote object
 freeing

On Wed, Jul 23, 2025 at 03:34:42PM +0200, Vlastimil Babka wrote:
> Since we don't control the NUMA locality of objects in percpu sheaves,
> allocations with node restrictions bypass them. Allocations without
> restrictions may however still expect to get local objects with high
> probability, and the introduction of sheaves can decrease it due to
> freed object from a remote node ending up in percpu sheaves.
> 
> The fraction of such remote frees seems low (5% on an 8-node machine)
> but it can be expected that some cache or workload specific corner cases
> exist. We can either conclude that this is not a problem due to the low
> fraction, or we can make remote frees bypass percpu sheaves and go
> directly to their slabs. This will make the remote frees more expensive,
> but if if's only a small fraction, most frees will still benefit from
> the lower overhead of percpu sheaves.
> 
> This patch thus makes remote object freeing bypass percpu sheaves,
> including bulk freeing, and kfree_rcu() via the rcu_free sheaf. However
> it's not intended to be 100% guarantee that percpu sheaves will only
> contain local objects. The refill from slabs does not provide that
> guarantee in the first place, and there might be cpu migrations
> happening when we need to unlock the local_lock. Avoiding all that could
> be possible but complicated so we can leave it for later investigation
> whether it would be worth it. It can be expected that the more selective
> freeing will itself prevent accumulation of remote objects in percpu
> sheaves so any such violations would have only short-term effects.
> 
> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
> ---
>  mm/slab_common.c |  7 +++++--
>  mm/slub.c        | 42 ++++++++++++++++++++++++++++++++++++------
>  2 files changed, 41 insertions(+), 8 deletions(-)
> 
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index 2d806e02568532a1000fd3912db6978e945dcfa8..f466f68a5bd82030a987baf849a98154cd48ef23 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -1623,8 +1623,11 @@ static bool kfree_rcu_sheaf(void *obj)
>  
>  	slab = folio_slab(folio);
>  	s = slab->slab_cache;
> -	if (s->cpu_sheaves)
> -		return __kfree_rcu_sheaf(s, obj);
> +	if (s->cpu_sheaves) {
> +		if (likely(!IS_ENABLED(CONFIG_NUMA) ||
> +			   slab_nid(slab) == numa_node_id()))
> +			return __kfree_rcu_sheaf(s, obj);
> +	}

This should be numa_mem_id() to handle memory-less NUMA nodes as
Christoph mentioned [1]?

I saw you addressed this in most of places but not this one.

With that addressed, please feel free to add:
Reviewed-by: Harry Yoo <harry.yoo@...cle.com>

[1] https://lore.kernel.org/linux-mm/c60ae681-6027-0626-8d4e-5833982bf1f0@gentwo.org

>  
>  	return false;
>  }

-- 
Cheers,
Harry / Hyeonggon