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

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Message-ID: <e341d9ef-9ec3-47c6-b5f0-3749c930f477@suse.cz>
Date: Thu, 22 Jan 2026 09:42:31 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Harry Yoo <harry.yoo@...cle.com>
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 1/22/26 08:02, Harry Yoo wrote:
> 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.
> 
> Hmm one more question.
> 
> Given frees to remote nodes bypass sheaves layer anyway, isn't it
> more reasonable to let refill_objects() fail sometimes instead of
> allocating new local slabs and fall back to slowpath (based on defrag_ratio)?

You mean if we can't refill from local partial list, we give up and perhaps
fail alloc_from_pcs()? Then the __slab_alloc_node() fallback would do
allocate local slab or try remote nodes?

Wouldn't that mean __slab_alloc_node() does all that work for a single
object, and slow everything down? Only in case of a new slab it would
somehow amortize because the next attempt would refill from it.

>> 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>
>> ---
> 
>