igc: Get rid of spurious interrupts

When running the igc with XDP/ZC in busy polling mode with deferral of hard
interrupts, interrupts still happen from time to time. That is caused by
the igc task watchdog which triggers Rx interrupts periodically.

That mechanism has been introduced to overcome skb/memory allocation
failures [1]. So the Rx clean functions stop processing the Rx ring in case
of such failure. The task watchdog triggers Rx interrupts periodically in
the hope that memory became available in the mean time.

The current behavior is undesirable for real time applications, because the
driver induced Rx interrupts trigger also the softirq processing. However,
all real time packets should be processed by the application which uses the
busy polling method.

Therefore, only trigger the Rx interrupts in case of real allocation
failures. Introduce a new flag for signaling that condition.

[1] - https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git/commit/?id=3be507547e6177e5c808544bd6a2efa2c7f1d436Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Acked-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Acked-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Kurt Kanzenbach <kurt@linutronix.de>
Reviewed-by: Simon Horman <horms@kernel.org>
Tested-by: Mor Bar-Gabay <morx.bar.gabay@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>

drivers/net/ethernet/intel/igc/igc.h

@@ -687,6 +687,7 @@ enum igc_ring_flags_t {
 	IGC_RING_FLAG_TX_DETECT_HANG,
 	IGC_RING_FLAG_AF_XDP_ZC,
 	IGC_RING_FLAG_TX_HWTSTAMP,
+	IGC_RING_FLAG_RX_ALLOC_FAILED,
 };
 
 #define ring_uses_large_buffer(ring) \

drivers/net/ethernet/intel/igc/igc_main.c

@@ -2191,6 +2191,7 @@ static bool igc_alloc_mapped_page(struct igc_ring *rx_ring,
 	page = dev_alloc_pages(igc_rx_pg_order(rx_ring));
 	if (unlikely(!page)) {
 		rx_ring->rx_stats.alloc_failed++;
+		set_bit(IGC_RING_FLAG_RX_ALLOC_FAILED, &rx_ring->flags);
 		return false;
 	}
 
@@ -2207,6 +2208,7 @@ static bool igc_alloc_mapped_page(struct igc_ring *rx_ring,
 		__free_page(page);
 
 		rx_ring->rx_stats.alloc_failed++;
+		set_bit(IGC_RING_FLAG_RX_ALLOC_FAILED, &rx_ring->flags);
 		return false;
 	}
 
@@ -2658,6 +2660,7 @@ static int igc_clean_rx_irq(struct igc_q_vector *q_vector, const int budget)
 		if (!skb) {
 			rx_ring->rx_stats.alloc_failed++;
 			rx_buffer->pagecnt_bias++;
+			set_bit(IGC_RING_FLAG_RX_ALLOC_FAILED, &rx_ring->flags);
 			break;
 		}
 
@@ -2738,6 +2741,7 @@ static void igc_dispatch_skb_zc(struct igc_q_vector *q_vector,
 	skb = igc_construct_skb_zc(ring, xdp);
 	if (!skb) {
 		ring->rx_stats.alloc_failed++;
+		set_bit(IGC_RING_FLAG_RX_ALLOC_FAILED, &ring->flags);
 		return;
 	}
 
@@ -5807,12 +5811,30 @@ static void igc_watchdog_task(struct work_struct *work)
 	if (adapter->flags & IGC_FLAG_HAS_MSIX) {
 		u32 eics = 0;
 
-		for (i = 0; i < adapter->num_q_vectors; i++)
-			eics |= adapter->q_vector[i]->eims_value;
+		for (i = 0; i < adapter->num_q_vectors; i++) {
+			struct igc_q_vector *q_vector = adapter->q_vector[i];
+			struct igc_ring *rx_ring;
+
+			if (!q_vector->rx.ring)
+				continue;
+
+			rx_ring = adapter->rx_ring[q_vector->rx.ring->queue_index];
+
+			if (test_bit(IGC_RING_FLAG_RX_ALLOC_FAILED, &rx_ring->flags)) {
+				eics |= q_vector->eims_value;
+				clear_bit(IGC_RING_FLAG_RX_ALLOC_FAILED, &rx_ring->flags);
+			}
+		}
+		if (eics)
 			wr32(IGC_EICS, eics);
 	} else {
+		struct igc_ring *rx_ring = adapter->rx_ring[0];
+
+		if (test_bit(IGC_RING_FLAG_RX_ALLOC_FAILED, &rx_ring->flags)) {
+			clear_bit(IGC_RING_FLAG_RX_ALLOC_FAILED, &rx_ring->flags);
 			wr32(IGC_ICS, IGC_ICS_RXDMT0);
 		}
+	}
 
 	igc_ptp_tx_hang(adapter);