Re: [PATCH v3] cgroup/bpf: fast path skb BPF filtering

[Pavel Begunkov]

On 12/15/21 22:07, Stanislav Fomichev wrote:
> On Wed, Dec 15, 2021 at 11:55 AM Pavel Begunkov <asml.silence@xxxxxxxxx> wrote:
> > On 12/15/21 19:15, Stanislav Fomichev wrote:
> > > On Wed, Dec 15, 2021 at 10:54 AM Pavel Begunkov <asml.silence@xxxxxxxxx> wrote:
> > > > On 12/15/21 18:24, sdf@xxxxxxxxxx wrote:
[...]
> > > > > I can probably do more experiments on my side once your patch is
> > > > > accepted. I'm mostly concerned with getsockopt(TCP_ZEROCOPY_RECEIVE).
> > > > > If you claim there is visible overhead for a direct call then there
> > > > > should be visible benefit to using CGROUP_BPF_TYPE_ENABLED there as
> > > > > well.
> > > >
> > > > Interesting, sounds getsockopt might be performance sensitive to
> > > > someone.
> > > >
> > > > FWIW, I forgot to mention that for testing tx I'm using io_uring
> > > > (for both zc and not) with good submission batching.
> > >
> > > Yeah, last time I saw 2-3% as well, but it was due to kmalloc, see
> > > more details in 9cacf81f8161, it was pretty visible under perf.
> > > That's why I'm a bit skeptical of your claims of direct calls being
> > > somehow visible in these 2-3% (even skb pulls/pushes are not 2-3%?).
> >
> > migrate_disable/enable together were taking somewhat in-between
> > 1% and 1.5% in profiling, don't remember the exact number. The rest
> > should be from rcu_read_lock/unlock() in BPF_PROG_RUN_ARRAY_CG_FLAGS()
> > and other extra bits on the way.
>
> You probably have a preemptiple kernel and preemptible rcu which most
> likely explains why you see the overhead and I won't (non-preemptible
> kernel in our env, rcu_read_lock is essentially a nop, just a compiler
> barrier).

Right. For reference tried out non-preemptible, perf shows the function
taking 0.8% with a NIC and 1.2% with a dummy netdev.


> > I'm skeptical I'll be able to measure inlining one function,
> > variability between boots/runs is usually greater and would hide it.
>
> Right, that's why I suggested to mirror what we do in set/getsockopt
> instead of the new extra CGROUP_BPF_TYPE_ENABLED. But I'll leave it up
> to you, Martin and the rest.

--
Pavel Begunkov