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@xxxxxxxxxx>
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 can also try to add something like BUILD_BUG_ON(sizeof(struct
tcp_zerocopy_receive) < BPF_SOCKOPT_KERN_BUF_SIZE) to make sure this
buffer gets adjusted whenever we touch tcp_zerocopy_receive.
