Subject: Re: [PATCH] net/smc: avoid atomic_set and smp_wmb in the tx path when possible

From: Dust Li <[email-masked]>
Date: Thu, 2 Nov 2023 22:54:18 +0800

On Thu, Nov 02, 2023 at 05:27:12PM +0800, Li RongQing wrote:
>these is less opportunity that conn->tx_pushing is not 1, since

these -> there ?

>tx_pushing is just checked with 1, so move the setting tx_pushing
>to 1 after atomic_dec_and_test() return false, to avoid atomic_set
>and smp_wmb in tx path when possible

The patch should add [PATCH net-next] subject-prefix since this is an optimization.

Besides, do you have any performance number ?

Thanks

>
>Signed-off-by: Li RongQing <[email-masked]>
>---
> net/smc/smc_tx.c | 7 ++++---
> 1 file changed, 4 insertions(+), 3 deletions(-)
>
>diff --git a/net/smc/smc_tx.c b/net/smc/smc_tx.c
>index 3b0ff3b..72dbdee 100644
>--- a/net/smc/smc_tx.c
>+++ b/net/smc/smc_tx.c
>@@ -667,8 +667,6 @@ int smc_tx_sndbuf_nonempty(struct smc_connection *conn)
> 		return 0;
> 
> again:
>-	atomic_set(&conn->tx_pushing, 1);
>-	smp_wmb(); /* Make sure tx_pushing is 1 before real send */
> 	rc = __smc_tx_sndbuf_nonempty(conn);
> 
> 	/* We need to check whether someone else have added some data into
>@@ -677,8 +675,11 @@ int smc_tx_sndbuf_nonempty(struct smc_connection *conn)
> 	 * If so, we need to push again to prevent those data hang in the send
> 	 * queue.
> 	 */
>-	if (unlikely(!atomic_dec_and_test(&conn->tx_pushing)))
>+	if (unlikely(!atomic_dec_and_test(&conn->tx_pushing))) {
>+		atomic_set(&conn->tx_pushing, 1);
>+		smp_wmb(); /* Make sure tx_pushing is 1 before real send */
> 		goto again;
>+	}
> 
> 	return rc;
> }
>-- 
>2.9.4