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

[Harry Yoo <harry.yoo@oracle.com>]

On Wed, Dec 03, 2025 at 07:15:12PM +0800, Hao Li wrote:
> 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?

Let's first see if Vlastimil had an intention, and...

> > > 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.

I'm not still sure that either way is superior, as it really depends on
the alloc/free pattern. If the CPU keeps allocating more objects, keeping
the empty sheaf is unnecessary, but we don't know what the alloc/free
pattern will be.

So strong opinion from me, but I think it'd be better make
__pcs_replace_{full,empty}_main() handle it consistently,
if there is no special intention.

> 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@gmail.com>

-- 
Cheers,
Harry / Hyeonggon