Re: [PATCH bpf-next 0/5] bpf: BPF specific memory allocator.

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Tue, Jun 28, 2022 at 7:35 PM Christoph Lameter <cl@gentwo.de> wrote:
>
> On Tue, 28 Jun 2022, Alexei Starovoitov wrote:
>
> > > That is a relatively new feature due to RT logic support. without RT this
> > > would be a simple irq disable.
> >
> > Not just RT.
> > It's a slow path:
> >         if (IS_ENABLED(CONFIG_PREEMPT_RT) ||
> >             unlikely(!object || !slab || !node_match(slab, node))) {
> >               local_unlock_irqrestore(&s->cpu_slab->lock,...);
> > and that's not the only lock in there.
> > new_slab->allocate_slab... alloc_pages grabbing more locks.
>
>
> Its not a lock for !RT.
>
> The fastpath is lockless if hardware allows that but then we go into more
> and more serialiation needs as the allocation gets more into the page
> allocator logic.

On RT fast path == slow path with a lock.
On !RT fast path is lock less.
That's all correct.
bpf side has to make sure safety in all possible paths
therefore RT or !RT makes no difference.