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

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Message-ID: <CAADnVQ+8x2b5qddRxU50xeq69XMY5RNi8ZfvTbERidKwTYrzqA@mail.gmail.com>
Date: Wed, 29 Oct 2025 17:24:15 -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 Wed, Oct 29, 2025 at 3:44 PM Vlastimil Babka <vbabka@...e.cz> wrote:
>
>
> You mean the one that doesn't go the "if (unlikely(slab != c->slab))" way?
> Well that unlikely() there might be quite misleading. It will be true when
> free follows shortly after alloc. If not, c->slab can be exhausted and
> replaced with a new one. Or the process is migrated to another cpu before
> freeing. The probability of slab == c->slab staying true drops quickly.
>
> So if your tests were doing frees shortly after alloc, you would be indeed
> hitting it reliably, but is it representative?
> However sheaves should work reliably as well too with such a pattern, so if
> some real code really does that significantly, it will not regress.

I see. The typical usage of bpf map on the tracing side is
to attach two bpf progs to begin/end of something (like function entry/exit),
then map_update() on entry that allocates an element, populate
with data, then consume this data in 2nd bpf prog on exit
that deletes the element.
So alloc/free happen in a quick succession on the same cpu.
This is, of course, just one of use cases, but it was the dominant
one during early days.