Re: [PATCH net-next 2/2] sctp: add support for MSG_MORE

[Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>]

On Fri, Feb 24, 2017 at 02:16:02AM +0800, Xin Long wrote:
> On Fri, Feb 24, 2017 at 1:40 AM, Marcelo Ricardo Leitner
> <marcelo.leitner@gmail.com> wrote:
> > On Thu, Feb 23, 2017 at 04:04:10PM +0000, David Laight wrote:
> >> From: Xin Long
> >> > Sent: 23 February 2017 03:46
> >> > On Tue, Feb 21, 2017 at 10:27 PM, David Laight <David.Laight@aculab.com> wrote:
> >> > > From: Xin Long
> >> > >> Sent: 18 February 2017 17:53
> >> > >> This patch is to add support for MSG_MORE on sctp.
> >> > >>
> >> > >> It adds force_delay in sctp_datamsg to save MSG_MORE, and sets it after
> >> > >> creating datamsg according to the send flag. sctp_packet_can_append_data
> >> > >> then uses it to decide if the chunks of this msg will be sent at once or
> >> > >> delay it.
> >> > >>
> >> > >> Note that unlike [1], this patch saves MSG_MORE in datamsg, instead of
> >> > >> in assoc. As sctp enqueues the chunks first, then dequeue them one by
> >> > >> one. If it's saved in assoc,the current msg's send flag (MSG_MORE) may
> >> > >> affect other chunks' bundling.
> >> > >
> >> > > I thought about that and decided that the MSG_MORE flag on the last data
> >> > > chunk was the only one that mattered.
> >> > > Indeed looking at any others is broken.
> >> > >
> >> > > Consider what happens if you have two small chunks queued, the first
> >> > > with MSG_MORE set, the second with it clear.
> >> > >
> >> > > I think that sctp_outq_flush() will look at the first chunk and decide it
> >> > > doesn't need to do anything because sctp_packet_transmit_chunk()
> >> > > returns SCTP_XMIT_DELAY.
> >> > > The data chunk with MSG_MORE clear won't even be looked at.
> >> > > So the data will never be sent.
> >>
> >> > It's not that bad as you thought, in sctp_packet_can_append_data():
> >> > when inflight = 0 || sctp_sk(asoc->base.sk)->nodelay, the chunks
> >> > would be still sent out.
> >>
> >> One of us isn't understanding the other :-)
> >>
> >> IIRC sctp_packet_can_append_data() is called for the first queued
> >> data chunk in order to decide whether to generate a message that
> >
> > Perhaps here lies the source of the confusion?
> > sctp_packet_can_append_data() is called for all queued data chunks, and
> > not just the first one.
> >
> > sctp_outq_flush
> >   (retransmissions here, omitted for simplicity)
> >   /* Finally, transmit new packets.  */
> >   while ((chunk = sctp_outq_dequeue_data(q)) != NULL) {
> >     sctp_packet_transmit_chunk
> >       sctp_packet_append_chunk
> >         sctp_packet_can_append_data
> >         __sctp_packet_append_chunk
> >
> > So chunks are checked one by one.
> I think I got David's point.
> like, the queue is:
> 
> chunk3[null]-->chunk2 [msg_more]-->chunk1 [msg_more]
> 
> it dequeue from chunk1, once it returns SCTP_XMIT_DELAY
> chunk2, chunk3 will has no chance to dequeue, as it will
> goto: sctpflush_out in sctp_outq_flush(), But we are expecting
> to send all chunks.

Ahh yes, exactly.

> 
> >
> >> consists only of data chunks.
> >
> > That's not really its purpose. It's to check if it can append a data
> > chunk to the packet being prepared, while respecting asoc state, cwnd,
> > etc.
> >
> > HTH!
> >
> >   Marcelo
> >
> >> If it returns SCTP_XMIT_OK then a message is built collecting the
> >> rest of the queued data chunks (until the window fills).
> >>
> >> So if I send a message with MSG_MORE set (on an idle connection)
> >> SCTP_XMIT_DELAY is returned and a message isn't sent.
> >>
> >> I now send a second small message, this time with MSG_MORE clear.
> >> The message is queued, then the code looks to see if it can send anything.
> >>
> >> sctp_packet_can_append_data() is called for the first queued chunk.
> >> Since it has force_delay set SCTP_XMIT_DELAY is returned and no
> >> message is built.
> >> The second message isn't even looked at.
> >>
> >> > What MSG_MORE flag actually does is ignore inflight = 0 and
> >> > sctp_sk(asoc->base.sk)->nodelay to delay the chunks, but still
> >> > it has to respect the original logic (like !chunk->msg->can_delay
> >> > || !sctp_packet_empty(packet) || ...)
> >> >
> >> > To delay the chunks with MSG_MORE set even when inflight is 0
> >> > it especially important here for users.
> >>
> >> I'm not too worried about that.
> >> Sending the first message was a cheap way to ensure something got
> >> sent if the application lied and didn't send a subsequent message.
> >>
> >> The change has hit Linus's tree, I'll should be able to test that
> >> and confirm what I think is going on.
> >>
> >>       David
> >>
> --
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