Re: [PATCH v2] cgroup/bpf: fast path for not loaded skb BPF filtering

[Pavel Begunkov <asml.silence@gmail.com>]

On 12/14/21 19:14, Martin KaFai Lau wrote:
> On Tue, Dec 14, 2021 at 11:40:26AM +0000, Pavel Begunkov wrote:
>> On 12/14/21 07:27, Martin KaFai Lau wrote:
>>> On Sat, Dec 11, 2021 at 07:17:49PM +0000, Pavel Begunkov wrote:
>>>> cgroup_bpf_enabled_key static key guards from overhead in cases where
>>>> no cgroup bpf program of a specific type is loaded in any cgroup. Turn
>>>> out that's not always good enough, e.g. when there are many cgroups but
>>>> ones that we're interesting in are without bpf. It's seen in server
>>>> environments, but the problem seems to be even wider as apparently
>>>> systemd loads some BPF affecting my laptop.
>>>>
>>>> Profiles for small packet or zerocopy transmissions over fast network
>>>> show __cgroup_bpf_run_filter_skb() taking 2-3%, 1% of which is from
>>>> migrate_disable/enable(), and similarly on the receiving side. Also
>>>> got +4-5% of t-put for local testing.
>>> What is t-put?  throughput?
>>
>> yes
>>
>>> Local testing means sending to lo/dummy?
>>
>> yes, it was dummy specifically
> Thanks for confirming.
> 
> Please also put these details in the commit log.
> I was slow.  With only '%' as a unit, it took me a min to guess
> what t-put may mean ;)

I guess requests/s is a more natural metric for net. I anyway going
to resend, will reword it a bit.

>>>> +#define CGROUP_BPF_TYPE_ENABLED(sk, atype)				       \
>>> and change cgroup.c to directly use this instead, so
>>> everywhere holding a fullsock sk will use this instead
>>> of having two helpers for empty check.
>>
>> Why?
> As mentioned earlier, prefer to have one way to do the same thing
> for checking with a fullsock.
> 
>> CGROUP_BPF_TYPE_ENABLED can't be a function atm because of header
>> dependency hell, and so it'd kill some of typization, which doesn't add
>> clarity.
> I didn't mean to change it to a function.  I actually think,
> for the sk context, it should eventually be folded with the existing
> cgroup_bpf_enabled() macro because those are the tests to ensure
> there is bpf prog to run before proceeding.
> Need to audit about the non fullsock case. not sure yet.

btw, would be nice to rewrite helpers as inline functions, but
sock, cgroup, etc. are not defined in bpf-cgroup.h are can't be
included. May make sense e.g. not include bpf-cgroup.h in bpf.h
but to move some definitions like struct cgroup_bpf into
include/linux/cgroup-defs.h.
Though I'd rather leave it to someone with a better grasp on
BPF code base.


>> And also it imposes some extra overhead to *sockopt using
>> the first helper directly.
> I think it is unimportant unless it is measurable in normal
> use case.

I hope so


>> I think it's better with two of them.
> Ok. I won't insist.  There are atype that may not have sk, so
> a separate inline function for checking emptiness may eventually
> be useful there.

-- 
Pavel Begunkov