linux-kernel - Re: [PATCH v3 06/21] slab: introduce percpu sheaves bootstrap

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Message-ID: <83f2ee6e-f767-40da-8cf4-08c6c185b92b@suse.cz>
Date: Wed, 21 Jan 2026 11:54:20 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Hao Li <hao.li@...ux.dev>
Cc: Harry Yoo <harry.yoo@...cle.com>, Petr Tesarik <ptesarik@...e.com>,
 Christoph Lameter <cl@...two.org>, David Rientjes <rientjes@...gle.com>,
 Roman Gushchin <roman.gushchin@...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 06/21] slab: introduce percpu sheaves bootstrap

On 1/19/26 12:32, Hao Li wrote:
> On Fri, Jan 16, 2026 at 03:40:26PM +0100, Vlastimil Babka wrote:
>> Until now, kmem_cache->cpu_sheaves was !NULL only for caches with
>> sheaves enabled. Since we want to enable them for almost all caches,
>> it's suboptimal to test the pointer in the fast paths, so instead
>> allocate it for all caches in do_kmem_cache_create(). Instead of testing
>> the cpu_sheaves pointer to recognize caches (yet) without sheaves, test
>> kmem_cache->sheaf_capacity for being 0, where needed, using a new
>> cache_has_sheaves() helper.
>> 
>> However, for the fast paths sake we also assume that the main sheaf
>> always exists (pcs->main is !NULL), and during bootstrap we cannot
>> allocate sheaves yet.
>> 
>> Solve this by introducing a single static bootstrap_sheaf that's
>> assigned as pcs->main during bootstrap. It has a size of 0, so during
>> allocations, the fast path will find it's empty. Since the size of 0
>> matches sheaf_capacity of 0, the freeing fast paths will find it's
>> "full". In the slow path handlers, we use cache_has_sheaves() to
>> recognize that the cache doesn't (yet) have real sheaves, and fall back.
>> Thus sharing the single bootstrap sheaf like this for multiple caches
>> and cpus is safe.
>> 
>> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
>> ---
>>  mm/slub.c | 119 ++++++++++++++++++++++++++++++++++++++++++--------------------
>>  1 file changed, 81 insertions(+), 38 deletions(-)
>> 
> 
> Nit: would it make sense to also update "if (s->cpu_sheaves)" to
> cache_has_sheaves() in kvfree_rcu_barrier_on_cache(), for consistency?

Ack, will do. Thanks.