netdev - [PATCH net-next 5/7] net/smc: correct settings of RMB window update limit

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Date:   Tue,  1 Mar 2022 17:44:00 +0800
From:   Dust Li <dust.li@...ux.alibaba.com>
To:     Karsten Graul <kgraul@...ux.ibm.com>,
        Tony Lu <tonylu@...ux.alibaba.com>,
        Guangguan Wang <guangguan.wang@...ux.alibaba.com>
Cc:     davem@...emloft.net, kuba@...nel.org, netdev@...r.kernel.org,
        linux-s390@...r.kernel.org, linux-rdma@...r.kernel.org
Subject: [PATCH net-next 5/7] net/smc: correct settings of RMB window update limit

rmbe_update_limit is used to limit announcing receive
window updating too frequently. RFC7609 request a minimal
increase in the window size of 10% of the receive buffer
space. But current implementation used:

  min_t(int, rmbe_size / 10, SOCK_MIN_SNDBUF / 2)

and SOCK_MIN_SNDBUF / 2 == 2304 Bytes, which is almost
always less then 10% of the receive buffer space.

This causes the receiver always sending CDC message to
update its consumer cursor when it consumes more then 2K
of data. And as a result, we may encounter something like
"TCP silly window syndrome" when sending 2.5~8K message.

This patch fixes this using max(rmbe_size / 10, SOCK_MIN_SNDBUF / 2).

With this patch and SMC autocorking enabled, qperf 2K/4K/8K
tcp_bw test shows 45%/75%/40% increase in throughput respectively.

Signed-off-by: Dust Li <dust.li@...ux.alibaba.com>
---
 net/smc/smc_core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/smc/smc_core.c b/net/smc/smc_core.c
index 29525d03b253..1674b2549f8b 100644
--- a/net/smc/smc_core.c
+++ b/net/smc/smc_core.c
@@ -1988,7 +1988,7 @@ static struct smc_buf_desc *smc_buf_get_slot(int compressed_bufsize,
  */
 static inline int smc_rmb_wnd_update_limit(int rmbe_size)
 {
-	return min_t(int, rmbe_size / 10, SOCK_MIN_SNDBUF / 2);
+	return max_t(int, rmbe_size / 10, SOCK_MIN_SNDBUF / 2);
 }

 /* map an rmb buf to a link */
-- 
2.19.1.3.ge56e4f7