netdev - RE: [PATCH] net/smc: avoid atomic_set and smp

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Message-ID: <3a5eedd0376b4a29b351a1f43b81c384@baidu.com>
Date: Fri, 3 Nov 2023 04:56:00 +0000
From: "Li,Rongqing" <lirongqing@...du.com>
To: Wenjia Zhang <wenjia@...ux.ibm.com>
CC: "linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: RE: [PATCH] net/smc: avoid atomic_set and smp_wmb in the tx path when
 possible



> -----Original Message-----
> From: Wenjia Zhang <wenjia@...ux.ibm.com>
> Sent: Friday, November 3, 2023 4:42 AM
> To: Li,Rongqing <lirongqing@...du.com>
> Cc: linux-s390@...r.kernel.org; netdev@...r.kernel.org
> Subject: Re: [PATCH] net/smc: avoid atomic_set and smp_wmb in the tx path
> when possible
> 
> 
> 
> On 02.11.23 10:27, Li RongQing wrote:
> > these is less opportunity that conn->tx_pushing is not 1, since
> > 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
> >
> I think we should avoid to use argument like "less opportunity" in commit
> message. Because "less opportunity" does not mean "no opportunity". Once it
> occurs, does it mean that what the patch changes is useless or wrong?
> 

I will reword the message.
I think this is a question of probability. even tx_pushing is not 1, this is still not a problem, atomic_dec_and_test(&conn->tx_pushing) will return false, transmit will be looped again, and tx_pushing will be added at any time

> > Signed-off-by: Li RongQing <lirongqing@...du.com>
> > ---
> >   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;
> >   }
> I'm afraid that the *if* statement would never be true, without setting the
> value of &conn->tx_pushing firstly.

I think conn->tx_pushing do not need to be set in this condition, and this patch is trying to avoid setting it 

Thanks

-Li