Re: [PATCH v4 07/22] slab: introduce percpu sheaves bootstrap

Vlastimil Babka
2026-01-26T08:42:56Z

On 1/26/26 07:13, Hao Li wrote:
> On Fri, Jan 23, 2026 at 07:52:45AM +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.
>>
>> Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
>> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
>> ---
>> mm/slab.h | 12 ++++++
>> mm/slab_common.c | 2 +-
>> mm/slub.c | 123 ++++++++++++++++++++++++++++++++++++-------------------
>> 3 files changed, 95 insertions(+), 42 deletions(-)
>
> Tiny consistency nit: in kfree_rcu_sheaf(), there's a remaining "if
> (s->cpu_sheaves)" that could be replaced with "if (cache_has_sheaves(s))" for

Ah thanks.

> consistency. It's trivial, so no need to respin - happy to have it addressed
> opportunistically.

Actually we should remove it completely from the fastpath per the design, as
__kfree_rcu_sheaf() checks that. Will do.

> The rest looks great to me!
>
> Reviewed-by: Hao Li <hao.li@linux.dev>

Thanks!