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

[Maciej Fijalkowski <maciej.fijalkowski@intel.com>]

On Wed, Jun 12, 2024 at 11:24:53AM +0200, Kurt Kanzenbach wrote:
> 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@linutronix.de>

Acked-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>

> ---
>  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@linutronix.de>
>