Re: Three questions about busy poll

[Cong Wang <xiyou.wangcong@gmail.com>]

On Thu, Feb 14, 2019 at 4:39 PM Willem de Bruijn
<willemdebruijn.kernel@gmail.com> wrote:
>
> 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.


Ah, yeah, this explains my confusion. I thought busy_poll refers to
all polling related syscalls, that is select()/poll()/epoll(), it looks like
epoll() is so special here. Probably we need some clarification in
net.txt.


>
> > 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.


Good point, it's probably like struct eventpoll vs. struct epitem.

The reason why I am looking for a per-socket tuning is to minimize
the impact of setting busy_poll. I don't know if it is possible to somehow
make this per-socket via epoll interfaces, perhaps fundamentally
it is impossible?


>
> > 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).

That commit message is the only thing I can find too. I kinda
need a formal documentation in man page and hopefully
an example too.

Thanks for your explanations!