netdev - [PATCH iwl-next] igc: Get rid of spurious interrupts

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Message-Id: <20240611-igc_irq-v1-1-49763284cb57@linutronix.de>
Date: Wed, 12 Jun 2024 11:24:53 +0200
From: Kurt Kanzenbach <kurt@...utronix.de>
To: Jesse Brandeburg <jesse.brandeburg@...el.com>, 
 Tony Nguyen <anthony.l.nguyen@...el.com>
Cc: "David S. Miller" <davem@...emloft.net>, 
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, 
 Paolo Abeni <pabeni@...hat.com>, 
 Sebastian Andrzej Siewior <bigeasy@...utronix.de>, 
 Maciej Fijalkowski <maciej.fijalkowski@...el.com>, 
 Vinicius Costa Gomes <vinicius.gomes@...el.com>, 
 intel-wired-lan@...ts.osuosl.org, netdev@...r.kernel.org, 
 Kurt Kanzenbach <kurt@...utronix.de>
Subject: [PATCH iwl-next] 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=3be507547e6177e5c808544bd6a2efa2c7f1d436

Signed-off-by: Kurt Kanzenbach <kurt@...utronix.de>
---
 drivers/net/ethernet/intel/igc/igc.h      |  1 +
 drivers/net/ethernet/intel/igc/igc_main.c | 24 ++++++++++++++++++++----
 2 files changed, 21 insertions(+), 4 deletions(-)

diff --git a/drivers/net/ethernet/intel/igc/igc.h b/drivers/net/ethernet/intel/igc/igc.h
index 8b14c029eda1..7bfe5030e2c0 100644
--- a/drivers/net/ethernet/intel/igc/igc.h
+++ b/drivers/net/ethernet/intel/igc/igc.h
@@ -682,6 +682,7 @@ 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) \
diff --git a/drivers/net/ethernet/intel/igc/igc_main.c b/drivers/net/ethernet/intel/igc/igc_main.c
index 305e05294a26..e666739dfac7 100644
--- a/drivers/net/ethernet/intel/igc/igc_main.c
+++ b/drivers/net/ethernet/intel/igc/igc_main.c
@@ -2192,6 +2192,7 @@ 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;
 	}

@@ -2208,6 +2209,7 @@ 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;
 	}

@@ -2659,6 +2661,7 @@ 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;
 		}

@@ -2739,6 +2742,7 @@ 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;
 	}

@@ -5811,11 +5815,23 @@ 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;
-		wr32(IGC_EICS, eics);
+		for (i = 0; i < adapter->num_q_vectors; i++) {
+			struct igc_ring *rx_ring = adapter->rx_ring[i];
+
+			if (test_bit(IGC_RING_FLAG_RX_ALLOC_FAILED, &rx_ring->flags)) {
+				eics |= adapter->q_vector[i]->eims_value;
+				clear_bit(IGC_RING_FLAG_RX_ALLOC_FAILED, &rx_ring->flags);
+			}
+		}
+		if (eics)
+			wr32(IGC_EICS, eics);
 	} else {
-		wr32(IGC_ICS, IGC_ICS_RXDMT0);
+		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);

---
base-commit: bb678f01804ccaa861b012b2b9426d69673d8a84
change-id: 20240611-igc_irq-ccc1c8bc6890

Best regards,
-- 
Kurt Kanzenbach <kurt@...utronix.de>