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

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Date:   Thu, 16 Dec 2021 13:21:26 +0000
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Stanislav Fomichev <sdf@...gle.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 12/15/21 22:07, Stanislav Fomichev wrote:
> On Wed, Dec 15, 2021 at 11:55 AM Pavel Begunkov <asml.silence@...il.com> wrote:
>>
>> On 12/15/21 19:15, Stanislav Fomichev wrote:
>>> 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:
[...]
>>>>> 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