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

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Message-ID: <20211214191430.h36grlvnxeicuqea@kafai-mbp.dhcp.thefacebook.com>
Date:   Tue, 14 Dec 2021 11:14:30 -0800
From:   Martin KaFai Lau <kafai@...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>,
        Song Liu <songliubraving@...com>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] cgroup/bpf: fast path for not loaded skb BPF filtering

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 ;)

> > [ ... ]
> > 
> > > diff --git a/include/linux/bpf-cgroup.h b/include/linux/bpf-cgroup.h
> > > index 11820a430d6c..793e4f65ccb5 100644
> > > --- a/include/linux/bpf-cgroup.h
> > > +++ b/include/linux/bpf-cgroup.h
> > > @@ -219,11 +219,28 @@ int bpf_percpu_cgroup_storage_copy(struct bpf_map *map, void *key, void *value);
> > >   int bpf_percpu_cgroup_storage_update(struct bpf_map *map, void *key,
> > >   				     void *value, u64 flags);
> > > +static inline bool
> > > +__cgroup_bpf_prog_array_is_empty(struct cgroup_bpf *cgrp_bpf,
> > > +				 enum cgroup_bpf_attach_type type)
> > Lets remove this.
> > 
> > > +{
> > > +	struct bpf_prog_array *array = rcu_access_pointer(cgrp_bpf->effective[type]);
> > > +
> > > +	return array == &empty_prog_array.hdr;
> > > +}
> > > +
> > > +#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.

> 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 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.

> I could inline the second one, but it wouldn't have been pretty.
Leaving CGROUP_BPF_TYPE_ENABLED as macro is fine.