netdev - Re: Three questions about busy poll

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Date:   Mon, 18 Feb 2019 20:30:30 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     Alexander Duyck <alexander.duyck@...il.com>,
        Eric Dumazet <edumazet@...gle.com>,
        "Samudrala, Sridhar" <sridhar.samudrala@...el.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: Three questions about busy poll

On Fri, Feb 15, 2019 at 2:47 PM Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
>
> > > > 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?
>
> I think it may be possible. The way busy_read and busy_poll work
> in sock_poll is that the sum of all (per socket tunable) busy_read
> durations on the sockets in the pollset is ~bound by (global) busy_poll.


Good idea!!

I was actually thinking about checking sk->sk_ll_usec
ep_set_busy_poll_napi_id(). Your idea sounds better than mine.
Maybe we can do both together. :)


>
> The epoll implementation is restricted in the sense that it polls only
> on one napi_id at a time. Alongside setting ep->napi_id in
> ep_set_busy_poll_napi_id, we could also set a new ep field takes
> the min of the global busy_poll and sk->sk_ll_usec.
>
> Though I guess you want to be able to poll on a given pollset
> without setting the global sysctl_net_busy_poll at all? That
> would be a useful feature both for epoll and poll/select. But
> definitely requires refining net_busy_loop_on() to optionally
> take some state derived from the sockets in the (e)pollset.

Yes, or at least give some control to each application.
I was thinking about this:

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index a5d219d920e7..1b104c8bda5e 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -432,7 +432,7 @@ static inline void ep_set_busy_poll_napi_id(struct
epitem *epi)
                return;

        sk = sock->sk;
-       if (!sk)
+       if (!sk || !sk->sk_ll_usec)
                return;

        napi_id = READ_ONCE(sk->sk_napi_id);

With this change, busy_poll would rely on either busy_read or
SO_BUSY_POLL.

Does this make any sense?

Thanks.