[PATCH v2] usbnet: optimize usbnet_bh() to reduce CPU load

[Leesoo Ahn <lsahn@ooseel.net>]

The current source pushes skb into dev->done queue by calling
skb_queue_tail() and then pop it by calling skb_dequeue() to branch to
rx_cleanup state for freeing urb/skb in usbnet_bh(). It takes extra CPU
load, 2.21% (skb_queue_tail) as follows.

-   11.58%     0.26%  swapper          [k] usbnet_bh
   - 11.32% usbnet_bh
      - 6.43% skb_dequeue
           6.34% _raw_spin_unlock_irqrestore
      - 2.21% skb_queue_tail
           2.19% _raw_spin_unlock_irqrestore
      - 1.68% consume_skb
         - 0.97% kfree_skbmem
              0.80% kmem_cache_free
           0.53% skb_release_data

To reduce the extra CPU load use return values jumping to rx_cleanup
state directly to free them instead of calling skb_queue_tail() and
skb_dequeue() for push/pop respectively.

-    7.87%     0.25%  swapper          [k] usbnet_bh
   - 7.62% usbnet_bh
      - 4.81% skb_dequeue
           4.74% _raw_spin_unlock_irqrestore
      - 1.75% consume_skb
         - 0.98% kfree_skbmem
              0.78% kmem_cache_free
           0.58% skb_release_data
        0.53% smsc95xx_rx_fixup

Signed-off-by: Leesoo Ahn <lsahn@ooseel.net>
---
v2:
  - Replace goto label with return statement to reduce goto entropy
  - Add CPU load information by perf in commit message

v1 at:
  https://patchwork.kernel.org/project/netdevbpf/patch/20221217161851.829497-1-lsahn@ooseel.net/
---
 drivers/net/usb/usbnet.c | 19 +++++++++----------
 1 file changed, 9 insertions(+), 10 deletions(-)

diff --git a/drivers/net/usb/usbnet.c b/drivers/net/usb/usbnet.c
index 64a9a80b2309..6e82fef90dd9 100644
--- a/drivers/net/usb/usbnet.c
+++ b/drivers/net/usb/usbnet.c
@@ -555,32 +555,30 @@ static int rx_submit (struct usbnet *dev, struct urb *urb, gfp_t flags)
 
 /*-------------------------------------------------------------------------*/
 
-static inline void rx_process (struct usbnet *dev, struct sk_buff *skb)
+static inline int rx_process(struct usbnet *dev, struct sk_buff *skb)
 {
 	if (dev->driver_info->rx_fixup &&
 	    !dev->driver_info->rx_fixup (dev, skb)) {
 		/* With RX_ASSEMBLE, rx_fixup() must update counters */
 		if (!(dev->driver_info->flags & FLAG_RX_ASSEMBLE))
 			dev->net->stats.rx_errors++;
-		goto done;
+		return 1;
 	}
 	// else network stack removes extra byte if we forced a short packet
 
 	/* all data was already cloned from skb inside the driver */
 	if (dev->driver_info->flags & FLAG_MULTI_PACKET)
-		goto done;
+		return 1;
 
 	if (skb->len < ETH_HLEN) {
 		dev->net->stats.rx_errors++;
 		dev->net->stats.rx_length_errors++;
 		netif_dbg(dev, rx_err, dev->net, "rx length %d\n", skb->len);
-	} else {
-		usbnet_skb_return(dev, skb);
-		return;
+		return 1;
 	}
 
-done:
-	skb_queue_tail(&dev->done, skb);
+	usbnet_skb_return(dev, skb);
+	return 0;
 }
 
 /*-------------------------------------------------------------------------*/
@@ -1528,13 +1526,14 @@ static void usbnet_bh (struct timer_list *t)
 		entry = (struct skb_data *) skb->cb;
 		switch (entry->state) {
 		case rx_done:
-			entry->state = rx_cleanup;
-			rx_process (dev, skb);
+			if (rx_process(dev, skb))
+				goto cleanup;
 			continue;
 		case tx_done:
 			kfree(entry->urb->sg);
 			fallthrough;
 		case rx_cleanup:
+cleanup:
 			usb_free_urb (entry->urb);
 			dev_kfree_skb (skb);
 			continue;
-- 
2.34.1