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Date: Wed, 16 Feb 2022 14:58:32 +0100
From: Stefan Raspl <raspl@...ux.ibm.com>
To: Dust Li <dust.li@...ux.alibaba.com>,
Karsten Graul <kgraul@...ux.ibm.com>,
Tony Lu <tonylu@...ux.alibaba.com>
Cc: kuba@...nel.org, davem@...emloft.net, netdev@...r.kernel.org,
linux-s390@...r.kernel.org, linux-rdma@...r.kernel.org
Subject: Re: [PATCH] net/smc: Add autocork support
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 <dust.li@...ux.alibaba.com>
> ---
> 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?
>
> /***************************** 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?
> +/* 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?
> + */
> +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.
Ciao,
Stefan
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