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

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Date:   Wed, 15 Dec 2021 11:15:05 -0800
From:   Stanislav Fomichev <sdf@...gle.com>
To:     Pavel Begunkov <asml.silence@...il.com>
Cc:     netdev@...r.kernel.org, bpf@...r.kernel.org,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] cgroup/bpf: fast path skb BPF filtering

On Wed, Dec 15, 2021 at 10:54 AM Pavel Begunkov <asml.silence@...il.com> wrote:
>
> On 12/15/21 18:24, sdf@...gle.com wrote:
> > On 12/15, Pavel Begunkov wrote:
> >> On 12/15/21 17:33, sdf@...gle.com wrote:
> >> > On 12/15, Pavel Begunkov wrote:
> >> > > On 12/15/21 16:51, sdf@...gle.com 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%?).
But tbf I don't understand how it all plays out with the io_uring.

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