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

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Date:   Tue, 14 Dec 2021 09:54:16 -0800
From:   sdf@...gle.com
To:     Pavel Begunkov <asml.silence@...il.com>
Cc:     Martin KaFai Lau <kafai@...com>, 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: [BPF PATCH for-next] cgroup/bpf: fast path for not loaded skb BPF filtering

On 12/11, Pavel Begunkov wrote:
> On 12/11/21 01:56, Martin KaFai Lau wrote:
> > On Sat, Dec 11, 2021 at 01:15:05AM +0000, Pavel Begunkov wrote:
> > > On 12/11/21 00:38, Martin KaFai Lau wrote:
> > > > On Fri, Dec 10, 2021 at 02:23:34AM +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.
> > > > >
> > > > > Signed-off-by: Pavel Begunkov <asml.silence@...il.com>
> > > > > ---
> > > > >    include/linux/bpf-cgroup.h | 24 +++++++++++++++++++++---
> > > > >    kernel/bpf/cgroup.c        | 23 +++++++----------------
> > > > >    2 files changed, 28 insertions(+), 19 deletions(-)
> > > > >
> > > > > diff --git a/include/linux/bpf-cgroup.h  
> b/include/linux/bpf-cgroup.h
> > > > > index 11820a430d6c..99b01201d7db 100644
> > > > > --- a/include/linux/bpf-cgroup.h
> > > > > +++ b/include/linux/bpf-cgroup.h
> > > > > @@ -141,6 +141,9 @@ struct cgroup_bpf {
> > > > >    	struct list_head progs[MAX_CGROUP_BPF_ATTACH_TYPE];
> > > > >    	u32 flags[MAX_CGROUP_BPF_ATTACH_TYPE];
> > > > > +	/* for each type tracks whether effective prog array is not  
> empty */
> > > > > +	unsigned long enabled_mask;
> > > > > +
> > > > >    	/* list of cgroup shared storages */
> > > > >    	struct list_head storages;
> > > > > @@ -219,11 +222,25 @@ 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_type_enabled(struct cgroup_bpf  
> *cgrp_bpf,
> > > > > +					     enum cgroup_bpf_attach_type atype)
> > > > > +{
> > > > > +	return test_bit(atype, &cgrp_bpf->enabled_mask);
> > > > > +}
> > > > > +
> > > > > +#define CGROUP_BPF_TYPE_ENABLED(sk, atype)				       \
> > > > > +({									       \
> > > > > +	struct cgroup *__cgrp =  
> sock_cgroup_ptr(&(sk)->sk_cgrp_data);	       \
> > > > > +									       \
> > > > > +	__cgroup_bpf_type_enabled(&__cgrp->bpf, (atype));		       \
> > > > > +})
> > > > I think it should directly test if the array is empty or not  
> instead of
> > > > adding another bit.
> > > >
> > > > Can the existing __cgroup_bpf_prog_array_is_empty(cgrp, ...) test  
> be used instead?
> > >
> > > That was the first idea, but it's still heavier than I'd wish.  
> 0.3%-0.7%
> > > in profiles, something similar in reqs/s. rcu_read_lock/unlock() pair  
> is
> > > cheap but anyway adds 2 barrier()s, and with bitmasks we can inline
> > > the check.
> > It sounds like there is opportunity to optimize
> > __cgroup_bpf_prog_array_is_empty().
> >
> > How about using rcu_access_pointer(), testing with  
> &empty_prog_array.hdr,
> > and then inline it?  The cgroup prog array cannot be all
> > dummy_bpf_prog.prog.  If that could be the case, it should be replaced
> > with &empty_prog_array.hdr earlier, so please check.

> I'd need to expose and export empty_prog_array, but that should do.
> Will try it out, thanks

Note that we already use __cgroup_bpf_prog_array_is_empty in
__cgroup_bpf_run_filter_setsockopt/__cgroup_bpf_run_filter_getsockopt
for exactly the same purpose. If you happen to optimize it, pls
update these places as well.