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

[Pavel Begunkov]

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:
> > > On 12/15, Pavel Begunkov wrote:
> > > > On 12/15/21 17:33, sdf@xxxxxxxxxx wrote:
> > > > > On 12/15, Pavel Begunkov wrote:
> > > > > > On 12/15/21 16:51, sdf@xxxxxxxxxx wrote:
> > > > > > > On 12/15, Pavel Begunkov wrote:
> > > > > > > > � /* Wrappers for __cgroup_bpf_run_filter_skb() guarded by cgroup_bpf_enabled. */
> > > > > > > > � #define BPF_CGROUP_RUN_PROG_INET_INGRESS(sk, skb)����������������� \
> > > > > > > > � ({����������������������������������������� \
> > > > > > > > ����� int __ret = 0;��������������������������������� \
> > > > > > > > -��� if (cgroup_bpf_enabled(CGROUP_INET_INGRESS))������������� \
> > > > > > > > +��� if (cgroup_bpf_enabled(CGROUP_INET_INGRESS) && sk &&������������� \
> > > > > > > > +������� CGROUP_BPF_TYPE_ENABLED((sk), CGROUP_INET_INGRESS))���������� \
> > > > > > >
> > > > > > > Why not add this __cgroup_bpf_run_filter_skb check to
> > > > > > > __cgroup_bpf_run_filter_skb? Result of sock_cgroup_ptr() is already there
> > > > > > > and you can use it. Maybe move the things around if you want
> > > > > > > it to happen earlier.
> > > > >
> > > > > > For inlining. Just wanted to get it done right, otherwise I'll likely be
> > > > > > returning to it back in a few months complaining that I see measurable
> > > > > > overhead from the function call :)
> > > > >
> > > > > Do you expect that direct call to bring any visible overhead?
> > > > > Would be nice to compare that inlined case vs
> > > > > __cgroup_bpf_prog_array_is_empty inside of __cgroup_bpf_run_filter_skb
> > > > > while you're at it (plus move offset initialization down?).
> > >
> > > > Sorry but that would be waste of time. I naively hope it will be visible
> > > > with net at some moment (if not already), that's how it was with io_uring,
> > > > that's what I see in the block layer. And in anyway, if just one inlined
> > > > won't make a difference, then 10 will.
> > >
> > > 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.

I'm skeptical I'll be able to measure inlining one function,
variability between boots/runs is usually greater and would hide it.

> But tbf I don't understand how it all plays out with the io_uring.

1 syscall per N requests (N=32 IIRC), 1 fdget() per N, no payload
page referencing (for zc), and so on


> (mostly trying to understand where there is some gain left on the
> table for TCP_ZEROCOPY_RECEIVE).

--
Pavel Begunkov