Re: [PATCH v4 9/9] mm, slub: skip percpu sheaves for remote object freeing

[Harry Yoo <harry.yoo@oracle.com>]

On Fri, Apr 25, 2025 at 10:27:29AM +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.
> 
> Another possible optimization to investigate is whether it would be
> beneficial for node-restricted or strict_numa allocations to attempt to
> obtain an object from percpu sheaves if the node or mempolicy (i.e.
> MPOL_LOCAL) happens to want the local node of the allocating cpu. Right
> now such allocations bypass sheaves, but they could probably look first
> whether the first available object in percpu sheaves is local, and with
> high probability succeed - and only bypass the sheaves in cases it's
> not local.
> 
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> ---
>  mm/slab_common.c |  7 +++++--
>  mm/slub.c        | 43 +++++++++++++++++++++++++++++++++++++------
>  2 files changed, 42 insertions(+), 8 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index cc273cc45f632e16644355831132cdc391219cec..2bf83e2b85b23f4db2b311edaded4bef6b7d01de 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -5924,8 +5948,15 @@ void slab_free(struct kmem_cache *s, struct slab *slab, void *object,
>  	if (unlikely(!slab_free_hook(s, object, slab_want_init_on_free(s), false)))
>  		return;
>  
> -	if (!s->cpu_sheaves || !free_to_pcs(s, object))
> -		do_slab_free(s, slab, object, object, 1, addr);
> +	if (s->cpu_sheaves) {
> +		if (likely(!IS_ENABLED(CONFIG_NUMA) ||
> +			   slab_nid(slab) == numa_node_id())) {
> +			free_to_pcs(s, object);

Shouldn't it call do_slab_free() when free_to_pcs() failed?

> +			return;
> +		}
> +	}
> +
> +	do_slab_free(s, slab, object, object, 1, addr);
>  }
>  
>  #ifdef CONFIG_MEMCG
> 
> -- 
> 2.49.0
> 
> 

-- 
Cheers,
Harry / Hyeonggon