Subject: Re: [PATCH net-next] sfc: reduce the number of requested xdp ev queues

From: Jesper Dangaard Brouer <[email-masked]>
Date: Tue, 15 Dec 2020 10:43:27 +0100

On Mon, 14 Dec 2020 17:29:06 -0800
Ivan Babrou <[email-masked]> wrote:

> Without this change the driver tries to allocate too many queues,
> breaching the number of available msi-x interrupts on machines
> with many logical cpus and default adapter settings:
> 
> Insufficient resources for 12 XDP event queues (24 other channels, max 32)
> 
> Which in turn triggers EINVAL on XDP processing:
> 
> sfc 0000:86:00.0 ext0: XDP TX failed (-22)

I have a similar QA report with XDP_REDIRECT:
  sfc 0000:05:00.0 ens1f0np0: XDP redirect failed (-22)

Here we are back to the issue we discussed with ixgbe, that NIC / msi-x
interrupts hardware resources are not enough on machines with many
logical cpus.

After this fix, what will happen if (cpu >= efx->xdp_tx_queue_count) ?
(Copied efx_xdp_tx_buffers code below signature)

The question leads to, does this driver need a fallback mechanism when
HW resource or systems logical cpus exceed the one TX-queue per CPU
assumption?

> Signed-off-by: Ivan Babrou <[email-masked]>
> ---
>  drivers/net/ethernet/sfc/efx_channels.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/ethernet/sfc/efx_channels.c
> b/drivers/net/ethernet/sfc/efx_channels.c index
> a4a626e9cd9a..1bfeee283ea9 100644 ---
> a/drivers/net/ethernet/sfc/efx_channels.c +++
> b/drivers/net/ethernet/sfc/efx_channels.c @@ -17,6 +17,7 @@
>  #include "rx_common.h"
>  #include "nic.h"
>  #include "sriov.h"
> +#include "workarounds.h"
>  
>  /* This is the first interrupt mode to try out of:
>   * 0 => MSI-X
> @@ -137,6 +138,7 @@ static int efx_allocate_msix_channels(struct
> efx_nic *efx, {
>  	unsigned int n_channels = parallelism;
>  	int vec_count;
> +	int tx_per_ev;
>  	int n_xdp_tx;
>  	int n_xdp_ev;
>  
> @@ -149,9 +151,9 @@ static int efx_allocate_msix_channels(struct
> efx_nic *efx,
>  	 * multiple tx queues, assuming tx and ev queues are both
>  	 * maximum size.
>  	 */
> -
> +	tx_per_ev = EFX_MAX_EVQ_SIZE / EFX_TXQ_MAX_ENT(efx);
>  	n_xdp_tx = num_possible_cpus();
> -	n_xdp_ev = DIV_ROUND_UP(n_xdp_tx, EFX_MAX_TXQ_PER_CHANNEL);
> +	n_xdp_ev = DIV_ROUND_UP(n_xdp_tx, tx_per_ev);
>  
>  	vec_count = pci_msix_vec_count(efx->pci_dev);
>  	if (vec_count < 0)

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer

/* Transmit a packet from an XDP buffer
 *
 * Returns number of packets sent on success, error code otherwise.
 * Runs in NAPI context, either in our poll (for XDP TX) or a different NIC
 * (for XDP redirect).
 */
int efx_xdp_tx_buffers(struct efx_nic *efx, int n, struct xdp_frame **xdpfs,
		       bool flush)
{
	struct efx_tx_buffer *tx_buffer;
	struct efx_tx_queue *tx_queue;
	struct xdp_frame *xdpf;
	dma_addr_t dma_addr;
	unsigned int len;
	int space;
	int cpu;
	int i;

	cpu = raw_smp_processor_id();

	if (!efx->xdp_tx_queue_count ||
	    unlikely(cpu >= efx->xdp_tx_queue_count))
		return -EINVAL;

	tx_queue = efx->xdp_tx_queues[cpu];
	if (unlikely(!tx_queue))
		return -EINVAL;

	if (unlikely(n && !xdpfs))
		return -EINVAL;

	if (!n)
		return 0;

	/* Check for available space. We should never need multiple
	 * descriptors per frame.
	 */
	space = efx->txq_entries +
		tx_queue->read_count - tx_queue->insert_count;

	for (i = 0; i < n; i++) {
		xdpf = xdpfs[i];

		if (i >= space)
			break;

		/* We'll want a descriptor for this tx. */
		prefetchw(__efx_tx_queue_get_insert_buffer(tx_queue));

		len = xdpf->len;

		/* Map for DMA. */
		dma_addr = dma_map_single(&efx->pci_dev->dev,
					  xdpf->data, len,
					  DMA_TO_DEVICE);
		if (dma_mapping_error(&efx->pci_dev->dev, dma_addr))
			break;

		/*  Create descriptor and set up for unmapping DMA. */
		tx_buffer = efx_tx_map_chunk(tx_queue, dma_addr, len);
		tx_buffer->xdpf = xdpf;
		tx_buffer->flags = EFX_TX_BUF_XDP |
				   EFX_TX_BUF_MAP_SINGLE;
		tx_buffer->dma_offset = 0;
		tx_buffer->unmap_len = len;
		tx_queue->tx_packets++;
	}

	/* Pass mapped frames to hardware. */
	if (flush && i > 0)
		efx_nic_push_buffers(tx_queue);

	if (i == 0)
		return -EIO;

	efx_xdp_return_frames(n - i, xdpfs + i);

	return i;
}