Message-ID: <CAAXMevrc0Xmza_Zo_zjwgqGSqMFw66cMw_uysX7GWDO2Y4VrMw@mail.gmail.com>
Date: Tue, 9 Dec 2025 17:42:13 +0800
From: Hao Li <haoli.tcs@...il.com>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Harry Yoo <harry.yoo@...cle.com>, Suren Baghdasaryan <surenb@...gle.com>, 
	"Liam R. Howlett" <Liam.Howlett@...cle.com>, Christoph Lameter <cl@...two.org>, 
	David Rientjes <rientjes@...gle.com>, Roman Gushchin <roman.gushchin@...ux.dev>, 
	Uladzislau Rezki <urezki@...il.com>, Sidhartha Kumar <sidhartha.kumar@...cle.com>, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, rcu@...r.kernel.org, 
	maple-tree@...ts.infradead.org, Venkat Rao Bagalkote <venkat88@...ux.ibm.com>
Subject: Re: slub: add barn_get_full_sheaf() and refine empty-main sheaf replacement

On Tue, Dec 9, 2025 at 2:51 AM Vlastimil Babka <vbabka@...e.cz> wrote:
>
> On 12/7/25 14:59, Harry Yoo wrote:
> > 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...
>
> Hm I don't think I aimed to make this difference on purpose, but I didn't
> also aim to make the alloc/free paths completely symmetric.

Got it.

> Rather the goal
> was just to do what seemed the best option in each situation. And probably
> getting a full sheaf and populating spare never seemed to be an important
> case to warrant the extra code for a situation that's only transient after
> boot (see below).
>
> >> > > 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.
>
> As Harry suggests, that assumes a specific pattern where we exhaust main
> sheaf first and then we fill it fully back. But even then this can only
> happen once per cpu and then we have populated the spare and are very
> unlikely to run into this situation again.

I agree that my original patch was trying to optimize a rare pattern and
added more code than is justified.

>
> Also it's unlikely that full sheaves even exist in the barn during this
> early stage when we would request them. That assumes cpus behave differently
> and some have returned full sheaves to the barn before other cpus have
> consumed their first full sheaf and request another.
>
> More likely both barn_replace_empty_sheaf() and barn_get_empty_sheaf() will
> fail and we do alloc_full_sheaf(). And then... I think I can see an issue in
> __pcs_replace_empty_main() that's more likely to be suboptimal than the lack
> of symmetry you point out. When we reach the last part below "we can reach
> here only when gfpflags_allow_blocking..." and we have empty pcs->main, a
> full sheaf from alloc_full_sheaf() and no spare, we should be doing
> "pcs->spare = pcs->main" and not barn_put_empty_sheaf(). Right? This is what
> can delay populating the spare more likely I think.

Thanks, this suboptimal case makes sense to me.

>
> >> 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.
>
> Yeah.
>
> > 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.
>
> I'd rather see some numbers. But the suboptimality pointed out above is more
> obvious to me. Do you agree and want to send a patch? :)

Sure! I’ve prepared a smaller patch and will send it as v2.

>
> >> 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@...il.com>
> >
>

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