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

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Message-ID: <f4166712-9a1e-51a0-409d-b7df25a66c52@linux.ibm.com>
Date:   Mon, 14 Feb 2022 11:29:10 +0100
From:   Stefan Raspl <raspl@...ux.ibm.com>
To:     Tony Lu <tonylu@...ux.alibaba.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 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!

Ciao,
Stefan