ibmveth: Optimize poll rescheduling process

When the ibmveth driver processes less than the budget, it must call
napi_complete_done() to release the instance. This function will
return false if the driver should avoid rearming interrupts.
Previously, the driver was ignoring the return code of
napi_complete_done(). As a result, there were unnecessary calls to
enable the veth irq.
Therefore, use the return code napi_complete_done() to determine if
irq rearm is necessary.

Additionally, in the event that new data is received immediately after
rearming interrupts, rather than just rescheduling napi, also jump
back to the poll processing loop since we are already in the poll
function (and know that we did not expense all of budget).

This slight tweak results in a 15% increase in TCP_RR transaction rate
(320k to 370k txns). We can see the ftrace data supports this:
PREV: ibmveth_poll = 8818014.0 us / 182802.0 hits = AVG 48.24
NEW:  ibmveth_poll = 8082398.0 us / 191413.0 hits = AVG 42.22

Signed-off-by: Nick Child <[email protected]>
Reviewed-by: Shannon Nelson <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: Nick Child

drivers/net/ethernet/ibm/ibmveth.c

@@ -1337,6 +1337,7 @@ static int ibmveth_poll(struct napi_struct *napi, int budget)
 	unsigned long lpar_rc;
 	u16 mss = 0;
 
+restart_poll:
 	while (frames_processed < budget) {
 		if (!ibmveth_rxq_pending_buffer(adapter))
 			break;
@@ -1420,24 +1421,25 @@ static int ibmveth_poll(struct napi_struct *napi, int budget)
 
 	ibmveth_replenish_task(adapter);
 
-	if (frames_processed < budget) {
-		napi_complete_done(napi, frames_processed);
+	if (frames_processed == budget)
+		goto out;
 
-		/* We think we are done - reenable interrupts,
-		 * then check once more to make sure we are done.
-		 */
-		lpar_rc = h_vio_signal(adapter->vdev->unit_address,
-				       VIO_IRQ_ENABLE);
+	if (!napi_complete_done(napi, frames_processed))
+		goto out;
 
-		BUG_ON(lpar_rc != H_SUCCESS);
+	/* We think we are done - reenable interrupts,
+	 * then check once more to make sure we are done.
+	 */
+	lpar_rc = h_vio_signal(adapter->vdev->unit_address, VIO_IRQ_ENABLE);
+	BUG_ON(lpar_rc != H_SUCCESS);
 
-		if (ibmveth_rxq_pending_buffer(adapter) &&
-		    napi_schedule(napi)) {
-			lpar_rc = h_vio_signal(adapter->vdev->unit_address,
-					       VIO_IRQ_DISABLE);
-		}
+	if (ibmveth_rxq_pending_buffer(adapter) && napi_schedule(napi)) {
+		lpar_rc = h_vio_signal(adapter->vdev->unit_address,
+				       VIO_IRQ_DISABLE);
+		goto restart_poll;
 	}
 
+out:
 	return frames_processed;
 }