Re: [PATCH RFC 12/19] slab: remove the do_slab_free() fastpath

[Alexei Starovoitov]

On Wed, Oct 29, 2025 at 3:44 PM Vlastimil Babka <vbabka@xxxxxxx> 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.