linux-kernel - Re: [PATCH v4 10/22] slab: add optimized sheaf refill from partial list

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Message-ID: <aXsT0FSP5IGCXxOt@hyeyoo>
Date: Thu, 29 Jan 2026 17:01:20 +0900
From: Harry Yoo <harry.yoo@...cle.com>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Petr Tesarik <ptesarik@...e.com>, Christoph Lameter <cl@...two.org>,
        David Rientjes <rientjes@...gle.com>,
        Roman Gushchin <roman.gushchin@...ux.dev>, Hao Li <hao.li@...ux.dev>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Uladzislau Rezki <urezki@...il.com>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Alexei Starovoitov <ast@...nel.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-rt-devel@...ts.linux.dev,
        bpf@...r.kernel.org, kasan-dev@...glegroups.com
Subject: Re: [PATCH v4 10/22] slab: add optimized sheaf refill from partial
 list

On Fri, Jan 23, 2026 at 07:52:48AM +0100, Vlastimil Babka wrote:
> At this point we have sheaves enabled for all caches, but their refill
> is done via __kmem_cache_alloc_bulk() which relies on cpu (partial)
> slabs - now a redundant caching layer that we are about to remove.
> 
> The refill will thus be done from slabs on the node partial list.
> Introduce new functions that can do that in an optimized way as it's
> easier than modifying the __kmem_cache_alloc_bulk() call chain.
> 
> Introduce struct partial_bulk_context, a variant of struct
> partial_context that can return a list of slabs from the partial list
> with the sum of free objects in them within the requested min and max.
> 
> Introduce get_partial_node_bulk() that removes the slabs from freelist
> and returns them in the list. There is a racy read of slab->counters
> so make sure the non-atomic write in __update_freelist_slow() is not
> tearing.
> 
> Introduce get_freelist_nofreeze() which grabs the freelist without
> freezing the slab.
> 
> Introduce alloc_from_new_slab() which can allocate multiple objects from
> a newly allocated slab where we don't need to synchronize with freeing.
> In some aspects it's similar to alloc_single_from_new_slab() but assumes
> the cache is a non-debug one so it can avoid some actions. It supports
> the allow_spin parameter, which we always set true here, but the
> followup change will reuse the function in a context where it may be
> false.
> 
> Introduce __refill_objects() that uses the functions above to fill an
> array of objects. It has to handle the possibility that the slabs will
> contain more objects that were requested, due to concurrent freeing of
> objects to those slabs. When no more slabs on partial lists are
> available, it will allocate new slabs. It is intended to be only used
> in context where spinning is allowed, so add a WARN_ON_ONCE check there.
> 
> Finally, switch refill_sheaf() to use __refill_objects(). Sheaves are
> only refilled from contexts that allow spinning, or even blocking.
> 
> Reviewed-by: Suren Baghdasaryan <surenb@...gle.com>
> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
> ---
>  mm/slub.c | 293 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++-----
>  1 file changed, 272 insertions(+), 21 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 22acc249f9c0..142a1099bbc1 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -778,7 +786,8 @@ __update_freelist_slow(struct slab *slab, struct freelist_counters *old,
>  	slab_lock(slab);
>  	if (slab->freelist == old->freelist &&
>  	    slab->counters == old->counters) {
> -		slab->freelist = new->freelist;
> +		/* prevent tearing for the read in get_partial_node_bulk() */
> +		WRITE_ONCE(slab->freelist, new->freelist);
>  		slab->counters = new->counters;
>  		ret = true;
>  	}

Other than the above being WRITE_ONCE(slab->counters, new->counters),
Looks good to me,
Reviewed-by: Harry Yoo <harry.yoo@...cle.com>

-- 
Cheers,
Harry / Hyeonggon