Subject: Re: [PATCH net-next] net/smc: increase SMC_WR_BUF_CNT

From: Dust Li <[email-masked]>
Date: Sat, 26 Oct 2024 07:58:39 +0800

On 2024-10-25 09:46:19, Wenjia Zhang wrote:
>From: Halil Pasic <[email-masked]>
>
>The current value of SMC_WR_BUF_CNT is 16 which leads to heavy
>contention on the wr_tx_wait workqueue of the SMC-R linkgroup and its
>spinlock when many connections are  competing for the buffer. Currently
>up to 256 connections per linkgroup are supported.
>
>To make things worse when finally a buffer becomes available and
>smc_wr_tx_put_slot() signals the linkgroup's wr_tx_wait wq, because
>WQ_FLAG_EXCLUSIVE is not used all the waiters get woken up, most of the
>time a single one can proceed, and the rest is contending on the
>spinlock of the wq to go to sleep again.
>
>For some reason include/linux/wait.h does not offer a top level wrapper
>macro for wait_event with interruptible, exclusive and timeout. I did
>not spend too many cycles on thinking if that is even a combination that
>makes sense (on the quick I don't see why not) and conversely I
>refrained from making an attempt to accomplish the interruptible,
>exclusive and timeout combo by using the abstraction-wise lower
>level __wait_event interface.
>
>To alleviate the tx performance bottleneck and the CPU overhead due to
>the spinlock contention, let us increase SMC_WR_BUF_CNT to 256.

Hi,

Have you tested other values, such as 64? In our internal version, we
have used 64 for some time.

Increasing this to 256 will require a 36K continuous physical memory
allocation in smc_wr_alloc_link_mem(). Based on my experience, this may
fail on servers that have been running for a long time and have
fragmented memory.

    link->wr_rx_bufs = kcalloc(SMC_WR_BUF_CNT * 3, SMC_WR_BUF_SIZE,
                               GFP_KERNEL);

As we can see, the link->wr_rx_bufs will increase from 16*3*48 = 2,304
to 256*3*48=36,864 (1 page to 9 pages).

Best regards,
Dust

>
>Signed-off-by: Halil Pasic <[email-masked]>
>Reported-by: Nils Hoppmann <[email-masked]>
>Reviewed-by: Wenjia Zhang <[email-masked]>
>Signed-off-by: Wenjia Zhang <[email-masked]>
>---
> net/smc/smc_wr.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
>diff --git a/net/smc/smc_wr.h b/net/smc/smc_wr.h
>index f3008dda222a..81e772e241f3 100644
>--- a/net/smc/smc_wr.h
>+++ b/net/smc/smc_wr.h
>@@ -19,7 +19,7 @@
> #include "smc.h"
> #include "smc_core.h"
> 
>-#define SMC_WR_BUF_CNT 16	/* # of ctrl buffers per link */
>+#define SMC_WR_BUF_CNT 256	/* # of ctrl buffers per link */
> 
> #define SMC_WR_TX_WAIT_FREE_SLOT_TIME	(10 * HZ)
> 
>-- 
>2.43.0
>