Re: [REGRESSION] slab: replace cpu (partial) slabs with sheaves

["Harry Yoo (Oracle)" <harry@kernel.org>]

On Fri, Mar 27, 2026 at 08:58:36AM +0000, Ryan Roberts wrote:
> >>>>> On 3/26/26 13:43, Aishwarya Rambhadran wrote:
> >>> Right so there should be just the overhead of the extra is_vmalloc_addr()
> >>> test. Possibly also the call of kfree_rcu_sheaf() if it's not inlined.
> >>> I'd say it's something we can just accept? It seems this is a unit test
> >>> being used as a microbenchmark, so it can be very sensitive even to such
> >>> details, but it should be negligible in practice.
> >>
> >> The perf/syscall cases might be a bit more concerning though? (those tests are
> >> from "perf bench syscall fork|execve"). Yes they are microbenchmarks, but a 7%
> >> increased cost for fork seems like something we'd want to avoid if we can.
> > 
> > Sure, I tried to explain those in my first reply. Harry then linked to how
> > that explanation can be verified. Hopefully it's really the same reason.
> 
> Ahh sorry I missed your first email. We only added that benchmark from 6.19 so
> don't have results for earlier kernels, but I'll ask Aishu to run it for 6.17
> and 6.18 to see if the results correlate with your expectation.
> 
> But from a high level perspective, a 7% regression on fork is not ideal even if
> there was a 7% improvement in 6.18.

If that improvement comes from the number of objects cached per CPU,
I'm not sure if determining the default value (# of cached objs) based on
"a point when microbenchmarks stop improving" is a reasonable measure
because the default value affects all slab caches and will inevitably
increase overall memory usage.

Hopefully we could discuss what a reasonable heuristic that
"works for most situations" looks like, and allow users to tune it further
based on their needs.

As a side note, changing sheaf capacity at runtime is not supported yet
(I'm working on it) and targeting at least before the next LTS.

-- 
Cheers,
Harry / Hyeonggon