Re: [PATCH net-next] sfc: backport XDP EV queue sharing from the out-of-tree driver

[Martin Habets <habetsm.xilinx@gmail.com>]

On Thu, Dec 10, 2020 at 04:18:53PM -0800, Ivan Babrou wrote:
> Queue sharing behaviour already exists in the out-of-tree sfc driver,
> available under xdp_alloc_tx_resources module parameter.

This comment is not relevant for in-tree patches. I'd also like to
make clear that we never intend to upstream any module parameters.

> This avoids the following issue on machines with many cpus:
> 
> 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)

The code changes themselves are good.
The real limit that is hit here is with the number of MSI-X interrupts.
Reducing the number of event queues needed also reduces the number of
interrupts required, so this is a good thing.
Another way to get around this issue is to increase the number of
MSI-X interrupts allowed bu the NIC using the sfboot tool.

Best regards,
Martin

> 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)
> --
> 2.29.2

-- 
Martin Habets <habetsm.xilinx@gmail.com>