netdev - Re: [PATCH net-next 2/3] net/smc: Remove corked dealyed work

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Date:   Mon, 14 Feb 2022 20:10:17 +0800
From:   Tony Lu <tonylu@...ux.alibaba.com>
To:     Stefan Raspl <raspl@...ux.ibm.com>
Cc:     kgraul@...ux.ibm.com, kuba@...nel.org, davem@...emloft.net,
        netdev@...r.kernel.org, linux-s390@...r.kernel.org
Subject: Re: [PATCH net-next 2/3] net/smc: Remove corked dealyed work

On Mon, Feb 14, 2022 at 11:29:10AM +0100, Stefan Raspl wrote:
> On 2/11/22 10:10, Tony Lu wrote:
> > On Mon, Jan 31, 2022 at 08:40:47PM +0100, Stefan Raspl wrote:
> > > On 1/30/22 19:02, Tony Lu wrote:
> > > > Based on the manual of TCP_CORK [1] and MSG_MORE [2], these two options
> > > > have the same effect. Applications can set these options and informs the
> > > > kernel to pend the data, and send them out only when the socket or
> > > > syscall does not specify this flag. In other words, there's no need to
> > > > send data out by a delayed work, which will queue a lot of work.
> > > > 
> > > > This removes corked delayed work with SMC_TX_CORK_DELAY (250ms), and the
> > > > applications control how/when to send them out. It improves the
> > > > performance for sendfile and throughput, and remove unnecessary race of
> > > > lock_sock(). This also unlocks the limitation of sndbuf, and try to fill
> > > > it up before sending.
> > > > 
> > > > [1] https://linux.die.net/man/7/tcp
> > > > [2] https://man7.org/linux/man-pages/man2/send.2.html
> > > > 
> > > > Signed-off-by: Tony Lu <tonylu@...ux.alibaba.com>
> > > > ---
> > > >    net/smc/smc_tx.c | 15 ++++++---------
> > > >    1 file changed, 6 insertions(+), 9 deletions(-)
> > > > 
> > > > diff --git a/net/smc/smc_tx.c b/net/smc/smc_tx.c
> > > > index 7b0b6e24582f..9cec62cae7cb 100644
> > > > --- a/net/smc/smc_tx.c
> > > > +++ b/net/smc/smc_tx.c
> > > > @@ -31,7 +31,6 @@
> > > >    #include "smc_tracepoint.h"
> > > >    #define SMC_TX_WORK_DELAY	0
> > > > -#define SMC_TX_CORK_DELAY	(HZ >> 2)	/* 250 ms */
> > > >    /***************************** sndbuf producer *******************************/
> > > > @@ -237,15 +236,13 @@ int smc_tx_sendmsg(struct smc_sock *smc, struct msghdr *msg, size_t len)
> > > >    		if ((msg->msg_flags & MSG_OOB) && !send_remaining)
> > > >    			conn->urg_tx_pend = true;
> > > >    		if ((msg->msg_flags & MSG_MORE || smc_tx_is_corked(smc)) &&
> > > > -		    (atomic_read(&conn->sndbuf_space) >
> > > > -						(conn->sndbuf_desc->len >> 1)))
> > > > -			/* for a corked socket defer the RDMA writes if there
> > > > -			 * is still sufficient sndbuf_space available
> > > > +		    (atomic_read(&conn->sndbuf_space)))
> > > > +			/* for a corked socket defer the RDMA writes if
> > > > +			 * sndbuf_space is still available. The applications
> > > > +			 * should known how/when to uncork it.
> > > >    			 */
> > > > -			queue_delayed_work(conn->lgr->tx_wq, &conn->tx_work,
> > > > -					   SMC_TX_CORK_DELAY);
> > > > -		else
> > > > -			smc_tx_sndbuf_nonempty(conn);
> > > > +			continue;
> > > 
> > > In case we just corked the final bytes in this call, wouldn't this
> > > 'continue' prevent us from accounting the Bytes that we just staged to be
> > > sent out later in the trace_smc_tx_sendmsg() call below?
> > > 
> > > > +		smc_tx_sndbuf_nonempty(conn);
> > > >    		trace_smc_tx_sendmsg(smc, copylen);
> > > 
> > 
> > If the application send out the final bytes in this call, the
> > application should also clear MSG_MORE or TCP_CORK flag, this action is
> > required based on the manuals [1] and [2]. So it is safe to cork the data
> > if flag is setted, and continue to the next loop until application
> > clears the flag.
> 
> Yes, I understand. But trace_smc_tx_sendmsg(smc, copylen) should be called
> for each portion of data that we transmit, i.e. each time we run through
> this loop. That is because parameter copylen is reset during each iteration.
> Now your patch adds a 'continue', which prevents that trace_smc_tc... call
> from being made. Which means the information that 'copylen' Bytes were
> transferred is lost forever, and the accounting of tx Bytes is off by
> 'copylen' Bytes, I believe!

This makes sense to me. It shouldn't be ignored if data was corked. I
will fix it in the next patch.

Thank you,
Tony Lu