linux-kernel - Re: [PATCH v3 17/21] slab: refill sheaves from all nodes

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Message-ID: <aXGrQSOoG_6NdqNT@hyeyoo>
Date: Thu, 22 Jan 2026 13:44:49 +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 v3 17/21] slab: refill sheaves from all nodes

On Fri, Jan 16, 2026 at 03:40:37PM +0100, Vlastimil Babka wrote:
> __refill_objects() currently only attempts to get partial slabs from the
> local node and then allocates new slab(s). Expand it to trying also
> other nodes while observing the remote node defrag ratio, similarly to
> get_any_partial().
> 
> This will prevent allocating new slabs on a node while other nodes have
> many free slabs. It does mean sheaves will contain non-local objects in
> that case. Allocations that care about specific node will still be
> served appropriately, but might get a slowpath allocation.
> 
> Like get_any_partial() we do observe cpuset_zone_allowed(), although we
> might be refilling a sheaf that will be then used from a different
> allocation context.
> 
> We can also use the resulting refill_objects() in
> __kmem_cache_alloc_bulk() for non-debug caches. This means
> kmem_cache_alloc_bulk() will get better performance when sheaves are
> exhausted. kmem_cache_alloc_bulk() cannot indicate a preferred node so
> it's compatible with sheaves refill in preferring the local node.
> Its users also have gfp flags that allow spinning, so document that
> as a requirement.
> 
> Reviewed-by: Suren Baghdasaryan <surenb@...gle.com>
> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
> ---

Could this cause strict_numa to not work as intended when
the policy is MPOL_BIND?

alloc_from_pcs() has:
> #ifdef CONFIG_NUMA
>         if (static_branch_unlikely(&strict_numa) &&
>                          node == NUMA_NO_NODE) {
>
>                 struct mempolicy *mpol = current->mempolicy;
>
>                 if (mpol) {
>                         /*
>                          * Special BIND rule support. If the local node
>                          * is in permitted set then do not redirect
>                          * to a particular node.
>                          * Otherwise we apply the memory policy to get
>                          * the node we need to allocate on.
>                          */
>                         if (mpol->mode != MPOL_BIND ||
>                                         !node_isset(numa_mem_id(), mpol->nodes))

This assumes the sheaves contain (mostly, although it wasn't strictly
guaranteed) objects from local node, and this change breaks that
assumption.

So... perhaps remove "Special BIND rule support"?

>
>                                 node = mempolicy_slab_node(); 
>                 }
>         }
> #endif

Otherwise LGTM.

-- 
Cheers,
Harry / Hyeonggon