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Date:   Fri, 15 Feb 2019 11:18:29 -0800
From:   Cong Wang <>
To:     "Samudrala, Sridhar" <>
Cc:     Alexander Duyck <>,
        Eric Dumazet <>,
        Linux Kernel Network Developers <>
Subject: Re: Three questions about busy poll

On Thu, Feb 14, 2019 at 4:43 PM Samudrala, Sridhar
<> wrote:
> On 2/14/2019 12:15 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?
> epoll based busypoll is only based on global sysctl_net_busy_poll.
> busy_poll value is used with poll()/select()/epoll and and busy_read is used
> with socket recvmsg

Right, I was confused by what the term "poll" refers to.

> 2. Why there is no socket option for Clearly
> sysctl_net_busy_poll is global and SO_BUSY_POLL only works for
> Not sure if it is useful to make it a per socket option. I think it could
> be a per poll/epoll fd option

It is useful, however, from Willem's reply it looks like hard to push it
down from epoll() interface to each socket.

> 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?
> A app can create one worker thread per device queue and a worker thread
> for an incoming connection can be selected based on SO_INCOMING_NAPI_ID so that
> all connections coming on a queue are processed by the same thread. This will
> allow epoll from a thread to be associated with sockets that receive packets
> from a single queue allowing busy polling.

This information is very useful. It also requires each thread pinning to each
CPU/RX queue, right?

Anyway, I will add this information to socket.7 man page.


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