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Date:   Mon, 4 Jan 2021 13:01:03 -0800
From:   Martin KaFai Lau <kafai@...com>
To:     <sdf@...gle.com>
CC:     Song Liu <song@...nel.org>, Networking <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>
Subject: Re: [PATCH bpf-next 1/2] bpf: try to avoid kzalloc in
 cgroup/{s,g}etsockopt

On Thu, Dec 31, 2020 at 12:14:13PM -0800, sdf@...gle.com wrote:
> On 12/30, Martin KaFai Lau wrote:
> > On Mon, Dec 21, 2020 at 02:22:41PM -0800, Song Liu wrote:
> > > On Thu, Dec 17, 2020 at 9:24 AM Stanislav Fomichev <sdf@...gle.com>
> > wrote:
> > > >
> > > > When we attach a bpf program to cgroup/getsockopt any other
> > getsockopt()
> > > > syscall starts incurring kzalloc/kfree cost. While, in general, it's
> > > > not an issue, sometimes it is, like in the case of
> > TCP_ZEROCOPY_RECEIVE.
> > > > TCP_ZEROCOPY_RECEIVE (ab)uses getsockopt system call to implement
> > > > fastpath for incoming TCP, we don't want to have extra allocations in
> > > > there.
> > > >
> > > > Let add a small buffer on the stack and use it for small (majority)
> > > > {s,g}etsockopt values. I've started with 128 bytes to cover
> > > > the options we care about (TCP_ZEROCOPY_RECEIVE which is 32 bytes
> > > > currently, with some planned extension to 64 + some headroom
> > > > for the future).
> > >
> > > I don't really know the rule of thumb, but 128 bytes on stack feels
> > too big to
> > > me. I would like to hear others' opinions on this. Can we solve the
> > problem
> > > with some other mechanisms, e.g. a mempool?
> > It seems the do_tcp_getsockopt() is also having "struct
> > tcp_zerocopy_receive"
> > in the stack.  I think the buf here is also mimicking
> > "struct tcp_zerocopy_receive", so should not cause any
> > new problem.
> Good point!
> 
> > However, "struct tcp_zerocopy_receive" is only 40 bytes now.  I think it
> > is better to have a smaller buf for now and increase it later when the
> > the future needs in "struct tcp_zerocopy_receive" is also upstreamed.
> I can lower it to 64. Or even 40?
I think either is fine.  Both will need another cacheline on bpf_sockopt_kern.
128 is a bit too much without a clear understanding on what "some headroom
for the future" means.

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