Subject: Re: [PATCH net-next v2 2/2] net/smc: reduce TX slot contention with exclusive wait

From: Dust Li <[email-masked]>
Date: Mon, 8 Jun 2026 22:04:34 +0800

On 2026-05-28 16:48:19, D. Wythe wrote:
>smc_wr_tx_get_free_slot() waits for a free TX slot with
>wait_event_interruptible_timeout(). Since the wait_event family
>enqueues waiters as non-exclusive, wake_up() may wake multiple
>waiters even though only one can use the slot, causing
>thundering-herd contention when slots are scarce.
>
>Use an exclusive wait loop with prepare_to_wait_exclusive() so
>wake_up() wakes only one waiter per freed slot.
>smc_wr_wakeup_tx_wait() still uses wake_up_all() during link
>teardown, so teardown behavior is unchanged.
>
>Performance measured with netperf TCP_RR (63 flows, 200B write /
>1000B read, 60s duration):
>
>+-------------------------------+---------------+---------------+
>| smcr_max_conns_per_lgr        | 32            | 255           |
>|-------------------------------+---------------+---------------|
>| before                        | 4.85 Gb/s     | 657.95 Mb/s   |
>|-------------------------------+---------------+---------------|
>| after                         | 5.01 Gb/s     | 2.2 Gb/s      |
>+-------------------------------+---------------+---------------+
>
>Signed-off-by: D. Wythe <[email-masked]>
>---
> net/smc/smc_wr.c | 36 ++++++++++++++++++++++++++----------
> 1 file changed, 26 insertions(+), 10 deletions(-)
>
>diff --git a/net/smc/smc_wr.c b/net/smc/smc_wr.c
>index 130bc6c26fb3..3cb47f77130e 100644
>--- a/net/smc/smc_wr.c
>+++ b/net/smc/smc_wr.c
>@@ -153,9 +153,11 @@ int smc_wr_tx_get_free_slot(struct smc_link *link,
> 			    struct smc_rdma_wr **wr_rdma_buf,
> 			    struct smc_wr_tx_pend_priv **wr_pend_priv)
> {
>+	unsigned long timeout = SMC_WR_TX_WAIT_FREE_SLOT_TIME;
> 	struct smc_link_group *lgr = smc_get_lgr(link);
> 	struct smc_wr_tx_pend *wr_pend;
> 	u32 idx = link->wr_tx_cnt;
>+	DEFINE_WAIT(wait);
> 	int rc;
> 
> 	*wr_buf = NULL;
>@@ -165,17 +167,31 @@ int smc_wr_tx_get_free_slot(struct smc_link *link,
> 		if (rc)
> 			return rc;
> 	} else {
>-		rc = wait_event_interruptible_timeout(
>-			link->wr_tx_wait,
>-			!smc_link_sendable(link) ||
>-			lgr->terminating ||
>-			(smc_wr_tx_get_free_slot_index(link, &idx) != -EBUSY),
>-			SMC_WR_TX_WAIT_FREE_SLOT_TIME);
>-		if (!rc) {
>-			/* timeout - terminate link */
>-			smcr_link_down_cond_sched(link);
>-			return -EPIPE;
>+		rc = 0;
>+		for (;;) {
>+			prepare_to_wait_exclusive(&link->wr_tx_wait, &wait,
>+						  TASK_INTERRUPTIBLE);
>+			if (!smc_link_sendable(link) || lgr->terminating ||
>+			    smc_wr_tx_get_free_slot_index(link, &idx) != -EBUSY)
>+				break;
>+			timeout = schedule_timeout(timeout);
>+			/* re-check */
>+			if (!smc_link_sendable(link) || lgr->terminating ||
>+			    smc_wr_tx_get_free_slot_index(link, &idx) != -EBUSY)
>+				break;
>+			if (!timeout) {
>+				/* timeout - terminate link */
>+				smcr_link_down_cond_sched(link);
>+				break;
>+			}

The change itself looks correct to me. But I think we should probably define
a wait_event_interruptible_exclusive_timeout() helper in include/linux/wait.h
rather than open-coding it in smc ?

>+			if (signal_pending(current)) {
>+				rc = -ERESTARTSYS;
>+				break;
>+			}
> 		}

One more thing, seems we changed the signal here, it's better to add a comment
or note it in the commit message.

Best regards,
Dust