linux-kernel - Re: [PATCH RFC 12/19] slab: remove the do_slab

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Message-ID: <CAADnVQ+nAA5OeCbjskbrtgYbPR4Mp-MtOfeXoQE5LUgcZOawEQ@mail.gmail.com>
Date: Fri, 24 Oct 2025 15:32:14 -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 12/19] slab: remove the do_slab_free() fastpath

On Thu, Oct 23, 2025 at 6:53 AM Vlastimil Babka <vbabka@...e.cz> wrote:
> @@ -6444,8 +6316,13 @@ void kfree_nolock(const void *object)
>          * since kasan quarantine takes locks and not supported from NMI.
>          */
>         kasan_slab_free(s, x, false, false, /* skip quarantine */true);
> +       /*
> +        * __slab_free() can locklessly cmpxchg16 into a slab, but then it might
> +        * need to take spin_lock for further processing.
> +        * Avoid the complexity and simply add to a deferred list.
> +        */
>         if (!free_to_pcs(s, x, false))
> -               do_slab_free(s, slab, x, x, 0, _RET_IP_);
> +               defer_free(s, x);

That should be rare, right?
free_to_pcs() should have good chances to succeed,
and pcs->spare should be there for kmalloc sheaves?
So trylock failure due to contention in barn_get_empty_sheaf()
and in barn_replace_full_sheaf() should be rare.

But needs to be benchmarked, of course.
The current fast path cmpxchg16 in !RT is very reliable
in my tests. Hopefully this doesn't regress.