netdev - Three questions about busy poll

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Date:   Thu, 14 Feb 2019 12:15:31 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Alexander Duyck <alexander.duyck@...il.com>
Cc:     Eric Dumazet <edumazet@...gle.com>, sridhar.samudrala@...el.com,
        Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Three questions about busy poll

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?

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.

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?

Thanks!

1. https://www.kernel.org/doc/Documentation/sysctl/net.txt