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

Hao Li
2026-01-19T11:32:25Z

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@suse.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?

--
Thanks,
Hao