Re: Three questions about busy poll

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

On Thu, Feb 14, 2019 at 3:15 PM Cong Wang <xiyou.wangcong@gmail.com> wrote:
>
> Hello,
>
> While looking into the busy polling in Linux kernel, three questions
> come into my mind:
>
> 1. In the document[1], it claims sysctl.net.busy_poll depends on
> either SO_BUSY_POLL or sysctl.net.busy_read. However, from the code in
> ep_set_busy_poll_napi_id(), I don't see such a dependency. It simply
> checks sysctl_net_busy_poll and sk->sk_napi_id, but sk->sk_napi_id is
> always set as long as we enable CONFIG_NET_RX_BUSY_POLL. So what I am
> missing here?

That documentation refers to sock_poll. This does call sk_busy_loop
individually on each socket in the pollset and thus respects those values.
Epoll was added later, after both sock_poll and that documentation.

> 2. Why there is no socket option for sysctl.net.busy_poll? Clearly
> sysctl_net_busy_poll is global and SO_BUSY_POLL only works for
> sysctl.net.busy_read.

I guess because of how sock_poll works. In that case it is not needed.
The poll duration applies more to the pollset than any of the
individual sockets, too.

> 3. How is SO_INCOMING_NAPI_ID supposed to be used? I can't find any
> useful documents online. Any example or more detailed doc?

From the commit message of 6d4339028b35 ("net: Introduce
SO_INCOMING_NAPI_ID") it sounds like a sharding mechanism that
maintains flow affinity by sharding based on rxqueue (assuming that
something like RSS was used to ensure flow affinity in the first
place).