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

Jesper Dangaard Brouer
2020-12-15T09:45:45Z

On Mon, 14 Dec 2020 17:29:06 -0800
Ivan Babrou <ivan@cloudflare.com> 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 <ivan@cloudflare.com>
> ---
> 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;
}