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From: Cong Wang <xiyou.wangcong@gmail.com>
To: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Cc: Alexander Duyck <alexander.duyck@gmail.com>,
	Eric Dumazet <edumazet@google.com>,
	"Samudrala, Sridhar" <sridhar.samudrala@intel.com>,
	Linux Kernel Network Developers <netdev@vger.kernel.org>
Subject: Re: Three questions about busy poll
Date: Mon, 18 Feb 2019 20:30:30 -0800	[thread overview]
Message-ID: <CAM_iQpWtg6=qjQ=JFQyEONHujHX0BWEswejtc==RxocHj365uQ@mail.gmail.com> (raw)
In-Reply-To: <CA+FuTSc-Oq0XWowxDEYeZ+oT6SG8YZSu5nvzeOO5sUq3v51Lvg@mail.gmail.com>

On Fri, Feb 15, 2019 at 2:47 PM Willem de Bruijn
<willemdebruijn.kernel@gmail.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.

  reply	other threads:[~2019-02-19  4:30 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-02-14 20:15 Cong Wang
2019-02-15  0:39 ` Willem de Bruijn
2019-02-15 19:04   ` Cong Wang
2019-02-15 22:46     ` Willem de Bruijn
2019-02-19  4:30       ` Cong Wang [this message]
2019-02-19 17:04         ` Willem de Bruijn
     [not found] ` <bfaff1c6-1269-1c33-31e1-0a78f78d7214@intel.com>
2019-02-15 19:18   ` Cong Wang
2019-02-22 20:00     ` Samudrala, Sridhar

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