netdev - Re: [PATCH net-next] sctp: fix busy polling

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Message-ID: <CANn89iKMQqC3u9SqfVP-Dq4Za+nQ331Z4w_KZaEUZrmpLG4UKQ@mail.gmail.com>
Date: Wed, 3 Jan 2024 11:51:03 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Xin Long <lucien.xin@...il.com>
Cc: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org, eric.dumazet@...il.com, 
	Jacob Moroni <jmoroni@...gle.com>, Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
Subject: Re: [PATCH net-next] sctp: fix busy polling

On Fri, Dec 22, 2023 at 7:34 PM Xin Long <lucien.xin@...il.com> wrote:
>
> On Fri, Dec 22, 2023 at 12:05 PM Eric Dumazet <edumazet@...gle.com> wrote:
> >
> > On Fri, Dec 22, 2023 at 5:08 PM Xin Long <lucien.xin@...il.com> wrote:
> > >
> > > On Tue, Dec 19, 2023 at 12:00 PM Eric Dumazet <edumazet@...gle.com> wrote:
> > > >
> > > > Busy polling while holding the socket lock makes litle sense,
> > > > because incoming packets wont reach our receive queue.
> > > >
> > > > Fixes: 8465a5fcd1ce ("sctp: add support for busy polling to sctp protocol")
> > > > Reported-by: Jacob Moroni <jmoroni@...gle.com>
> > > > Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> > > > Cc: Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
> > > > Cc: Xin Long <lucien.xin@...il.com>
> > > > ---
> > > >  net/sctp/socket.c | 10 ++++------
> > > >  1 file changed, 4 insertions(+), 6 deletions(-)
> > > >
> > > > diff --git a/net/sctp/socket.c b/net/sctp/socket.c
> > > > index 5fb02bbb4b349ef9ab9c2790cccb30fb4c4e897c..6b9fcdb0952a0fe599ae5d1d1cc6fa9557a3a3bc 100644
> > > > --- a/net/sctp/socket.c
> > > > +++ b/net/sctp/socket.c
> > > > @@ -2102,6 +2102,10 @@ static int sctp_recvmsg(struct sock *sk, struct msghdr *msg, size_t len,
> > > >         if (unlikely(flags & MSG_ERRQUEUE))
> > > >                 return inet_recv_error(sk, msg, len, addr_len);
> > > >
> > > > +       if (sk_can_busy_loop(sk) &&
> > > > +           skb_queue_empty_lockless(&sk->sk_receive_queue))
> > > > +               sk_busy_loop(sk, flags & MSG_DONTWAIT);
> > > > +
> > > Here is no any sk_state check, if the SCTP socket(TCP type) has been
> > > already closed by peer, will sctp_recvmsg() block here?
> >
> > Busy polling is only polling the NIC queue, hoping to feed this socket
> > for incoming packets.
> OK, will it block if there's no incoming packets on the NIC queue?
>
> If yes, when sysctl net.core.busy_read=1, my concern is:
>
>      client                server
>      -------------------------------
>                            listen()
>      connect()
>                            accept()
>      close()
>                            recvmsg() <----
>
> recvmsg() is supposed to return right away as the connection is
> already close(). With this patch, will recvmsg() be able to do
> that if no more incoming packets in the NIC after close()?


Answer is yes for a variety of reasons :

net.core.busy_read=1 means :

Busy poll will happen for
1) at most one usec, and
2) as long as there is no packet in sk->sk_receive_queue (see
sk_busy_loop_end())

But busy poll is only started on sockets that had established packets.

A listener will not engage this because sk->sk_napi_id does not
contain a valid NAPI ID.



>
> Thanks.
>
> >
> > Using more than a lockless read of sk->sk_receive_queue is not really necessary,
> > and racy anyway.
> >
> > Eliezer Tamir added a check against sk_state for no good reason in
> > TCP, my plan is to remove it.
> >
> > There are other states where it still makes sense to allow busy polling.
> >
> >
> > >
> > > Maybe here it needs a `!(sk->sk_shutdown & RCV_SHUTDOWN)` check,
> > > which is set when it's closed by the peer.
> >
> > See above. Keep this as simple as possible...
> >
> >
> > >
> > > Thanks
> > >
> > > >         lock_sock(sk);
> > > >
> > > >         if (sctp_style(sk, TCP) && !sctp_sstate(sk, ESTABLISHED) &&
> > > > @@ -9046,12 +9050,6 @@ struct sk_buff *sctp_skb_recv_datagram(struct sock *sk, int flags, int *err)
> > > >                 if (sk->sk_shutdown & RCV_SHUTDOWN)
> > > >                         break;
> > > >
> > > > -               if (sk_can_busy_loop(sk)) {
> > > > -                       sk_busy_loop(sk, flags & MSG_DONTWAIT);
> > > > -
> > > > -                       if (!skb_queue_empty_lockless(&sk->sk_receive_queue))
> > > > -                               continue;
> > > > -               }
> > > >
> > > >                 /* User doesn't want to wait.  */
> > > >                 error = -EAGAIN;
> > > > --
> > > > 2.43.0.472.g3155946c3a-goog
> > > >