Subject: Re: [PATCH] net/smc: Add autocork support

From: "dust.li" <[email-masked]>
Date: Wed, 16 Feb 2022 23:27:21 +0800

On Wed, Feb 16, 2022 at 02:58:32PM +0100, Stefan Raspl wrote:
>On 2/16/22 04:49, Dust Li wrote:
>> This patch adds autocork support for SMC which could improve
>> throughput for small message by x2 ~ x4.
>> 
>> The main idea is borrowed from TCP autocork with some RDMA
>> specific modification:
>> 1. The first message should never cork to make sure we won't
>>     bring extra latency
>> 2. If we have posted any Tx WRs to the NIC that have not
>>     completed, cork the new messages until:
>>     a) Receive CQE for the last Tx WR
>>     b) We have corked enough message on the connection
>> 3. Try to push the corked data out when we receive CQE of
>>     the last Tx WR to prevent the corked messages hang in
>>     the send queue.
>> 
>> Both SMC autocork and TCP autocork check the TX completion
>> to decide whether we should cork or not. The difference is
>> when we got a SMC Tx WR completion, the data have been confirmed
>> by the RNIC while TCP TX completion just tells us the data
>> have been sent out by the local NIC.
>> 
>> Add an atomic variable tx_pushing in smc_connection to make
>> sure only one can send to let it cork more and save CDC slot.
>> 
>> SMC autocork should not bring extra latency since the first
>> message will always been sent out immediately.
>> 
>> The qperf tcp_bw test shows more than x4 increase under small
>> message size with Mellanox connectX4-Lx, same result with other
>> throughput benchmarks like sockperf/netperf.
>> The qperf tcp_lat test shows SMC autocork has not increase any
>> ping-pong latency.
>> 
>> BW test:
>>   client: smc_run taskset -c 1 qperf smc-server -oo msg_size:1:64K:*2 \
>> 			-t 30 -vu tcp_bw
>>   server: smc_run taskset -c 1 qperf
>> 
>> MsgSize(Bytes)        TCP         SMC-NoCork           SMC-AutoCork
>>        1         2.57 MB/s     698 KB/s(-73.5%)     2.98 MB/s(16.0% )
>>        2          5.1 MB/s    1.41 MB/s(-72.4%)     5.82 MB/s(14.1% )
>>        4         10.2 MB/s    2.83 MB/s(-72.3%)     11.7 MB/s(14.7% )
>>        8         20.8 MB/s    5.62 MB/s(-73.0%)     22.9 MB/s(10.1% )
>>       16         42.5 MB/s    11.5 MB/s(-72.9%)     45.5 MB/s(7.1%  )
>>       32         80.7 MB/s    22.3 MB/s(-72.4%)     86.7 MB/s(7.4%  )
>>       64          155 MB/s    45.6 MB/s(-70.6%)      160 MB/s(3.2%  )
>>      128          295 MB/s    90.1 MB/s(-69.5%)      273 MB/s(-7.5% )
>>      256          539 MB/s     179 MB/s(-66.8%)      610 MB/s(13.2% )
>>      512          943 MB/s     360 MB/s(-61.8%)     1.02 GB/s(10.8% )
>>     1024         1.58 GB/s     710 MB/s(-56.1%)     1.91 GB/s(20.9% )
>>     2048         2.47 GB/s    1.34 GB/s(-45.7%)     2.92 GB/s(18.2% )
>>     4096         2.86 GB/s     2.5 GB/s(-12.6%)      2.4 GB/s(-16.1%)
>>     8192         3.89 GB/s    3.14 GB/s(-19.3%)     4.05 GB/s(4.1%  )
>>    16384         3.29 GB/s    4.67 GB/s(41.9% )     5.09 GB/s(54.7% )
>>    32768         2.73 GB/s    5.48 GB/s(100.7%)     5.49 GB/s(101.1%)
>>    65536            3 GB/s    4.85 GB/s(61.7% )     5.24 GB/s(74.7% )
>> 
>> Latency test:
>>   client: smc_run taskset -c 1 qperf smc-server -oo msg_size:1:64K:*2 \
>> 			-t 30 -vu tcp_lat
>>   server: smc_run taskset -c 1 qperf
>> 
>>   MsgSize              SMC-NoCork           SMC-AutoCork
>>         1               9.7 us               9.6 us( -1.03%)
>>         2              9.43 us              9.39 us( -0.42%)
>>         4               9.6 us              9.35 us( -2.60%)
>>         8              9.42 us               9.2 us( -2.34%)
>>        16              9.13 us              9.43 us(  3.29%)
>>        32              9.19 us               9.5 us(  3.37%)
>>        64              9.38 us               9.5 us(  1.28%)
>>       128               9.9 us              9.29 us( -6.16%)
>>       256              9.42 us              9.26 us( -1.70%)
>>       512                10 us              9.45 us( -5.50%)
>>      1024              10.4 us               9.6 us( -7.69%)
>>      2048              10.4 us              10.2 us( -1.92%)
>>      4096                11 us              10.5 us( -4.55%)
>>      8192              11.7 us              11.8 us(  0.85%)
>>     16384              14.5 us              14.2 us( -2.07%)
>>     32768              19.4 us              19.3 us( -0.52%)
>>     65536              28.1 us              28.8 us(  2.49%)
>> 
>> With SMC autocork support, we can archive better throughput than
>> TCP in most message sizes without any latency tradeoff.
>> 
>> Signed-off-by: Dust Li <[email-masked]>
>> ---
>>   net/smc/smc.h     |   2 +
>>   net/smc/smc_cdc.c |  11 +++--
>>   net/smc/smc_tx.c  | 118 ++++++++++++++++++++++++++++++++++++++++------
>>   3 files changed, 114 insertions(+), 17 deletions(-)
>> 
>> diff --git a/net/smc/smc.h b/net/smc/smc.h
>> index a096d8af21a0..bc7df235281c 100644
>> --- a/net/smc/smc.h
>> +++ b/net/smc/smc.h
>> @@ -192,6 +192,8 @@ struct smc_connection {
>>   						 * - dec on polled tx cqe
>>   						 */
>>   	wait_queue_head_t	cdc_pend_tx_wq; /* wakeup on no cdc_pend_tx_wr*/
>> +	atomic_t		tx_pushing;     /* nr_threads trying tx push */
>> +
>>   	struct delayed_work	tx_work;	/* retry of smc_cdc_msg_send */
>>   	u32			tx_off;		/* base offset in peer rmb */
>> diff --git a/net/smc/smc_cdc.c b/net/smc/smc_cdc.c
>> index 9d5a97168969..2b37bec90824 100644
>> --- a/net/smc/smc_cdc.c
>> +++ b/net/smc/smc_cdc.c
>> @@ -48,9 +48,14 @@ static void smc_cdc_tx_handler(struct smc_wr_tx_pend_priv *pnd_snd,
>>   		conn->tx_cdc_seq_fin = cdcpend->ctrl_seq;
>>   	}
>> -	if (atomic_dec_and_test(&conn->cdc_pend_tx_wr) &&
>> -	    unlikely(wq_has_sleeper(&conn->cdc_pend_tx_wq)))
>> -		wake_up(&conn->cdc_pend_tx_wq);
>> +	if (atomic_dec_and_test(&conn->cdc_pend_tx_wr)) {
>> +		/* If this is the last pending WR complete, we must push to
>> +		 * prevent hang when autocork enabled.
>> +		 */
>> +		smc_tx_sndbuf_nonempty(conn);
>> +		if (unlikely(wq_has_sleeper(&conn->cdc_pend_tx_wq)))
>> +			wake_up(&conn->cdc_pend_tx_wq);
>> +	}
>>   	WARN_ON(atomic_read(&conn->cdc_pend_tx_wr) < 0);
>>   	smc_tx_sndbuf_nonfull(smc);
>> diff --git a/net/smc/smc_tx.c b/net/smc/smc_tx.c
>> index 5df3940d4543..bc737ac79805 100644
>> --- a/net/smc/smc_tx.c
>> +++ b/net/smc/smc_tx.c
>> @@ -31,6 +31,7 @@
>>   #include "smc_tracepoint.h"
>>   #define SMC_TX_WORK_DELAY	0
>> +#define SMC_DEFAULT_AUTOCORK_SIZE	(64 * 1024)
>
>Probably a matter of taste, but why not use hex here?

Yeah, I have no option on this, I will change it in the next version.
But I think it should have no real difference since the compiler
should do the calculation.

>
>>   /***************************** sndbuf producer *******************************/
>> @@ -127,10 +128,52 @@ static int smc_tx_wait(struct smc_sock *smc, int flags)
>>   static bool smc_tx_is_corked(struct smc_sock *smc)
>>   {
>>   	struct tcp_sock *tp = tcp_sk(smc->clcsock->sk);
>> -
>>   	return (tp->nonagle & TCP_NAGLE_CORK) ? true : false;
>>   }
>
>Can you drop this line elimination?

Sure, Will do it in the next version.

>
>
>> +/* If we have pending CDC messages, do not send:
>> + * Because CQE of this CDC message will happen shortly, it gives
>> + * a chance to coalesce future sendmsg() payload in to one RDMA Write,
>> + * without need for a timer, and with no latency trade off.
>> + * Algorithm here:
>> + *  1. First message should never cork
>> + *  2. If we have pending CDC messages, wait for the first
>> + *     message's completion
>> + *  3. Don't cork to much data in a single RDMA Write to prevent burst,
>> + *     total corked message should not exceed min(64k, sendbuf/2)
>
>I assume the 64k is incurred from IP as used by RoCEv2?

Acturally, 64K is from my previous experience with RoCEv2. We found in our
RoCE environment, When post large RDMA Write request, it is very easy to
trigger ECN/PFC and cause bandwidth drop with incast. And we add some flow
control in the software not to post single RDMA Write with large data to
alleviate that. 64K is a good value we found in our environment.

>
>
>> + */
>> +static bool smc_should_autocork(struct smc_sock *smc, struct msghdr *msg,
>> +				int size_goal)
>> +{
>> +	struct smc_connection *conn = &smc->conn;
>> +
>> +	if (atomic_read(&conn->cdc_pend_tx_wr) == 0 ||
>> +	    smc_tx_prepared_sends(conn) > min(size_goal,
>> +					      conn->sndbuf_desc->len >> 1))
>> +		return false;
>> +	return true;
>> +}
>> +
>> +static bool smc_tx_should_cork(struct smc_sock *smc, struct msghdr *msg)
>> +{
>> +	struct smc_connection *conn = &smc->conn;
>> +
>> +	if (smc_should_autocork(smc, msg, SMC_DEFAULT_AUTOCORK_SIZE))
>> +		return true;
>
>Are there any fixed plans to make SMC_DEFAULT_AUTOCORK dynamic...? 'cause
>otherwise we could simply eliminate this parameter, and use the define within
>smc_should_autocork() instead.

Yes! Actually I'd like it to be dynamic variable too...

I didn't do it because I also want to add a control switch for the autocork
feature just like TCP. In that case I need to add 2 variables here.
But I found adding dynamic variables using netlink would introduce a lot of
redundant code and may even bring ABI compatibility issues in the future, as
I mentioned here:
https://lore.kernel.org/netdev/[email-masked]/T/#mecfcd3f8c816d07dbe35e4748d17008331c89523

I'm not sure that's the right way to do it. In this case, I prefer using
sysctl which I think would be easier, but I would like to listen to your advice.

Thanks

>Ciao,
>Stefan