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Date:   Fri, 15 Feb 2019 11:04:38 -0800
From:   Cong Wang <>
To:     Willem de Bruijn <>
Cc:     Alexander Duyck <>,
        Eric Dumazet <>,,
        Linux Kernel Network Developers <>
Subject: Re: Three questions about busy poll

On Thu, Feb 14, 2019 at 4:39 PM Willem de Bruijn
<> wrote:
> On Thu, Feb 14, 2019 at 3:15 PM Cong Wang <> wrote:
> >
> > Hello,
> >
> > While looking into the busy polling in Linux kernel, three questions
> > come into my mind:
> >
> > 1. In the document[1], it claims depends on
> > either SO_BUSY_POLL or 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

> > 2. Why there is no socket option for Clearly
> > sysctl_net_busy_poll is global and SO_BUSY_POLL only works for
> >
> 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!

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