Re: [PATCH RFC 0/8] mm/slab: enable runtime sheaves tuning

[Pedro Falcato <pfalcato@suse.de>]

On Sat, May 16, 2026 at 01:24:24AM +0900, Harry Yoo (Oracle) wrote:
> Background
> ==========
> 
> Sheaves were introduced in v6.18, and starting from v7.0, they are
> enabled for all slab caches (except for kmem_cache{,_node}). In the
> pre-sheaves era, there was a cpu_partial parameter to tune the number
> of objects cached per CPU. However, sheaves don't have an equivalent
> and the sheaf capacity is determined in the kernel code.

What semantic do you need from this?

> 
> The goal is to allow tuning sheaves at runtime by the next LTS.
> 
> Overview
> ========
> 
> This patchset does two main things:
> 
>   1. Make the sheaf_capacity sysfs attribute writable so that the number
>      of objects cached per CPU can be changed at runtime, and
> 
>   2. Expose MAX_FULL_SHEAVES and MAX_EMPTY_SHEAVES as sysfs attributes
>      rather than constants, so that users can tune them.
> 
> Measuring the performance impact of these tunables is TBD.
> 
> Roughly, the sequence to change sheaf_capacity is as follows:
> 
>   1. Disable sheaves. Make all online CPUs replace their main sheaves
>      with the bootstrap sheaf under local_lock and wait for completion.

This is extremely destabilizing performance-wise, were I to guess.

> 
>   2. Wait for all in-flight RCU callbacks to be processed.

and this too.

>   
>   3. Flush and free all existing sheaves.
> 
>   4. Re-enable sheaves with a new capacity.
> 
> Challenges
> ==========
> 
> 1. Allocations and frees can happen concurrently at any point between
>    these steps, and we cannot introduce heavyweight synchronization
>    mechanisms on the fastpath.
> 
> 2. Currently, cache_has_sheaves() checks whether a cache has sheaves.
>    This works now because sheaves cannot be enabled or disabled once
>    the cache is created.
> 
>    The question "Does this cache has sheaves?" should be split into
>    "Does this cache support sheaves?" and "Does this CPU actually has
>     sheaves enabled right now?".
> 
> 3. Once the sheaf capacity update is complete, no sheaf with stale
>    capacity must remain.

Why? I don't see a huge problem with having multiple sheaves with different
capacities, as long as you adequately, opportunistically kill the sheaves
if they don't have the desired size (say, once a sheaf is fully empty).

-- 
Pedro