Re: Three questions about busy poll

From: "Samudrala, Sridhar" <sridhar.samudrala@intel.com>
To: Cong Wang <xiyou.wangcong@gmail.com>
Cc: Alexander Duyck <alexander.duyck@gmail.com>,
	Eric Dumazet <edumazet@google.com>,
	Linux Kernel Network Developers <netdev@vger.kernel.org>
Subject: Re: Three questions about busy poll
Date: Fri, 22 Feb 2019 12:00:19 -0800	[thread overview]
Message-ID: <a4eef6a6-6f44-996f-6b5f-aaa710405aab@intel.com> (raw)
In-Reply-To: <CAM_iQpXa_ra5Gi3Wiv+JRwixArozLDKOHHAdBym_3k1CzPYuaw@mail.gmail.com>


On 2/15/2019 11:18 AM, Cong Wang wrote:
> On Thu, Feb 14, 2019 at 4:43 PM Samudrala, Sridhar
> <sridhar.samudrala@intel.com> 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 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?
>>
>> 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 sysctl.net.busy_poll? Clearly
>> sysctl_net_busy_poll is global and SO_BUSY_POLL only works for
>> sysctl.net.busy_read.
>>
>>
>> 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?
Thread pinning will not be required if XPS is setup to use xps-rxqs that 
allows
configuring symmetric queues so that tx-queue is selected based on rx-queue.
See XPS using receive queues map in Documentation/networking/scaling.txt
>
> Anyway, I will add this information to socket.7 man page.
>
> Thanks!