netdev - Re: [RFC PATCH bpf-next 0/2] bpf: Implement shared persistent fast(er) sk

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Date:   Tue, 24 Aug 2021 09:03:20 -0700
From:   sdf@...gle.com
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     hjm2133@...umbia.edu, bpf@...r.kernel.org, netdev@...r.kernel.org,
        ppenkov@...gle.com
Subject: Re: [RFC PATCH bpf-next 0/2] bpf: Implement shared persistent
 fast(er) sk_storoage mode

On 08/23, Alexei Starovoitov wrote:
> On Mon, Aug 23, 2021 at 05:52:50PM -0400, Hans Montero wrote:
> > From: Hans Montero <hjm2133@...umbia.edu>
> >
> > This patch set adds a BPF local storage optimization. The first patch  
> adds the
> > feature, and the second patch extends the bpf selftests so that the  
> feature is
> > tested.
> >
> > We are running BPF programs for each egress packet and noticed that
> > bpf_sk_storage_get incurs a significant amount of cpu time. By inlining  
> the
> > storage into struct sock and accessing that instead of performing a map  
> lookup,
> > we expect to reduce overhead for our specific use-case.

> Looks like a hack to me. Please share the perf numbers and setup details.
> I think there should be a different way to address performance concerns
> without going into such hacks.

What kind of perf numbers would you like to see? What we see here is
that bpf_sk_storage_get() cycles are somewhere on par with hashtable
lookups (we've moved off of 5-tuple ht lookup to sk_storage). Looking
at the code, it seems it's mostly coming from the following:

   sk_storage = rcu_dereference(sk->sk_bpf_storage);
   sdata = rcu_dereference(local_storage->cache[smap->cache_idx]);
   return sdata->data

We do 3 cold-cache references :-( This is where the idea of inlining
something in the socket itself came from. The RFC is just to present
the case and discuss. I was thinking about doing some kind of
inlining at runtime (and fallback to non-inlined case) but wanted
to start with discussing this compile-time option first.

We can also try to save sdata somewhere in the socket to avoid two
lookups for the cached case, this can potentially save us two  
rcu_dereference's.
Is that something that looks acceptable? I was wondering whether you've
considered any socket storage optimizations on your side?

I can try to set up some office hours to discuss in person if that's
preferred.

> > This also has a
> > side-effect of persisting the socket storage, which can be beneficial.

> Without explicit opt-in such sharing will cause multiple bpf progs to  
> corrupt
> each other data.

New BPF_F_SHARED_LOCAL_STORAGE flag is here to provide this opt-in.