slub: add barn_get_full_sheaf() and refine empty-main sheaf replacement

[Hao Li]

On Wed, Dec 03, 2025 at 02:46:22PM +0900, Harry Yoo wrote:
> On Tue, Dec 02, 2025 at 05:00:08PM +0800, Hao Li wrote:
> > Introduce barn_get_full_sheaf(), a helper that detaches a full sheaf from
> > the per-node barn without requiring an empty sheaf in exchange.
> > 
> > Use this helper in __pcs_replace_empty_main() to change how an empty main
> > per-CPU sheaf is handled:
> > 
> >   - If pcs->spare is NULL and pcs->main is empty, first try to obtain a
> >     full sheaf from the barn via barn_get_full_sheaf(). On success, park
> >     the empty main sheaf in pcs->spare and install the full sheaf as the
> >     new pcs->main.
> > 
> >   - If pcs->spare already exists and has objects, keep the existing
> >     behavior of simply swapping pcs->main and pcs->spare.
> > 
> >   - Only when both pcs->main and pcs->spare are empty do we fall back to
> >     barn_replace_empty_sheaf() and trade the empty main sheaf into the
> >     barn in exchange for a full one.
> 
> Hi Hao,
> 
> Yeah this is a very subtle difference between __pcs_replace_full_main()
> and __pcs_replace_empty_main(), that the former installs the full main
> sheaf in pcs->spare, while the latter replaces the empty main sheaf with
> a full sheaf from the barn without populating pcs->spare.

Exactly.

> 
> Is it intentional, Vlastimil?
> 
> > This makes the empty-main path more symmetric with __pcs_replace_full_main(),
> > which for a full main sheaf parks the full sheaf in pcs->spare and pulls an
> > empty sheaf from the barn. It also matches the documented design more closely:
> > 
> >   "When both percpu sheaves are found empty during an allocation, an empty
> >    sheaf may be replaced with a full one from the per-node barn."
> 
> I'm not convinced that this change is worthwhile by adding more code;
> you probably need to make a stronger argument for why it should be done.

Hi Harry,

Let me explain my intuition in more detail.

Previously, when pcs->main was empty and pcs->spare was NULL, we used
barn_replace_empty_sheaf() to trade the empty main sheaf into the barn
in exchange for a full one. As a result, pcs->main became full, but
pcs->spare remained NULL. Later, when frees filled pcs->main again,
__pcs_replace_full_main() had to call into the barn to obtain an empty
sheaf, because there was still no local spare to use.

With this patch, when pcs->main is empty and pcs->spare is NULL,
__pcs_replace_empty_main() instead uses barn_get_full_sheaf() to pull a
full sheaf from the barn while keeping the now‑empty main sheaf locally
as pcs->spare. The next time pcs->main becomes full,
__pcs_replace_full_main() can simply swap main and spare, with no barn
operations and no need to allocate a new empty sheaf.

In other words, although we still need one barn operation when main
first becomes empty in __pcs_replace_empty_main(), we avoid a future
barn operation on the subsequent “main full” path in
__pcs_replace_full_main.

Thanks.

> 
> > Signed-off-by: Hao Li <haoli.tcs at gmail.com>
> > ---
> 
> -- 
> Cheers,
> Harry / Hyeonggon