linux-kernel - Re: [PATCH RFC 11/19] slab: remove SLUB_CPU

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Message-ID: <CAADnVQKBPF8g3JgbCrcGFx35Bujmta2vnJGM9pgpcLq1-wqLHg@mail.gmail.com>
Date: Fri, 24 Oct 2025 13:43:52 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Christoph Lameter <cl@...two.org>, 
	David Rientjes <rientjes@...gle.com>, Roman Gushchin <roman.gushchin@...ux.dev>, 
	Harry Yoo <harry.yoo@...cle.com>, Uladzislau Rezki <urezki@...il.com>, 
	"Liam R. Howlett" <Liam.Howlett@...cle.com>, Suren Baghdasaryan <surenb@...gle.com>, 
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>, Alexei Starovoitov <ast@...nel.org>, linux-mm <linux-mm@...ck.org>, 
	LKML <linux-kernel@...r.kernel.org>, linux-rt-devel@...ts.linux.dev, 
	bpf <bpf@...r.kernel.org>, kasan-dev <kasan-dev@...glegroups.com>
Subject: Re: [PATCH RFC 11/19] slab: remove SLUB_CPU_PARTIAL

On Thu, Oct 23, 2025 at 6:53 AM Vlastimil Babka <vbabka@...e.cz> wrote:
>
>  static bool has_pcs_used(int cpu, struct kmem_cache *s)
> @@ -5599,21 +5429,18 @@ static void __slab_free(struct kmem_cache *s, struct slab *slab,
>                 new.inuse -= cnt;
>                 if ((!new.inuse || !prior) && !was_frozen) {
>                         /* Needs to be taken off a list */
> -                       if (!kmem_cache_has_cpu_partial(s) || prior) {

I'm struggling to convince myself that it's correct.
Losing '|| prior' means that we will be grabbing
this "speculative" spin_lock much more often.
While before the change we need spin_lock only when
slab was partially empty
(assuming cpu_partial was on for caches where performance matters).

Also what about later check:
if (prior && !on_node_partial) {
       spin_unlock_irqrestore(&n->list_lock, flags);
       return;
}
and
if (unlikely(!prior)) {
                add_partial(n, slab, DEACTIVATE_TO_TAIL);

Say, new.inuse == 0 then 'n' will be set,
do we lose the slab?
Because before the change it would be added to put_cpu_partial() ?

but... since AI didn't find any bugs here, I must be wrong :)