Commit 229671ac authored by Eric Dumazet's avatar Eric Dumazet Committed by Jakub Kicinski
Browse files

net: remove cpu stall in txq_trans_update() 



txq_trans_update() currently uses txq->xmit_lock_owner
to conditionally update txq->trans_start.

For regular devices, txq->xmit_lock_owner is updated
from HARD_TX_LOCK() and HARD_TX_UNLOCK(), and this apparently
causes cpu stalls.

Using dev->lltx, which sits in a read-mostly cache-line,
and already used in HARD_TX_LOCK() and HARD_TX_UNLOCK()
helps cpu prediction.

On an AMD EPYC 7B12 dual socket server, tcp_rr with 128 threads
and 30,000 flows gets a 5 % increase in throughput.

As explained in commit 95ecba62 ("net: fix races in
netdev_tx_sent_queue()/dev_watchdog()") I am planning
to no longer update txq->trans_start in the fast path
in a followup patch.

Signed-off-by: default avatarEric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250408202742.2145516-1-edumazet@google.com


Signed-off-by: default avatarJakub Kicinski <kuba@kernel.org>
parent 9c056ec6

drivers/net/ethernet/ti/am65-cpsw-nuss.c

+1 −1
Original line number Diff line number Diff line
@@ -427,7 +427,7 @@ static void am65_cpsw_nuss_ndo_host_tx_timeout(struct net_device *ndev, 


	if (netif_tx_queue_stopped(netif_txq)) {

		/* try recover if stopped by us */

		txq_trans_update(netif_txq);

		txq_trans_update(ndev, netif_txq);

		netif_tx_wake_queue(netif_txq);

	}

}

include/linux/netdevice.h

+4 −3
Original line number Diff line number Diff line
@@ -4693,9 +4693,10 @@ static inline void __netif_tx_unlock_bh(struct netdev_queue *txq) 
/*

 * txq->trans_start can be read locklessly from dev_watchdog()

 */

static inline void txq_trans_update(struct netdev_queue *txq)

static inline void txq_trans_update(const struct net_device *dev,

				    struct netdev_queue *txq)

{

	if (txq->xmit_lock_owner != -1)

	if (!dev->lltx)

		WRITE_ONCE(txq->trans_start, jiffies);

}
@@ -5214,7 +5215,7 @@ static inline netdev_tx_t netdev_start_xmit(struct sk_buff *skb, struct net_devi 


	rc = __netdev_start_xmit(ops, skb, dev, more);

	if (rc == NETDEV_TX_OK)

		txq_trans_update(txq);

		txq_trans_update(dev, txq);



	return rc;

}