Re: [PATCH net-next 4/4] eth: mpnic: add a NAPI depletion check

[Harshitha Ramamurthy]

On Thu, Oct 1, 2026 at 9:41 AM Daniel Zahka <daniel.zahka@xxxxxxxxx> wrote:
>
> The Rx path can wedge if page pool allocations fail in
> __mpnic_fill_bdq(). The problematic sequence is: page pool allocations
> fail, bdq rings are empty, device receives packet from the network, but
> drops it for lack of buffer space and does not raise interrupt,
> mpnic_poll() does not run, so page pool allocations are not retried.
>
> To break out of this cycle, we can add a depletion check to our service
> task. The depletion check checks if posted buffers are under a low
> watermark, and raises a completion queue interrupt, so that mpnic_poll
> will run on the corresponding napi vector.
>
> A note about the lock free reads: We don't bother synchronizing the ring
> head and tail reads with mpnic_poll(), because a false positive signal
> would simply raise a spurious interrupt scheduling mpnic_poll(), and a
> false negative signal will result in a retry, where if mpnic_poll() has
> truly not been running, should then report accurately whether or not
> posted Rx buffers are low.
>
> Signed-off-by: Daniel Zahka <daniel.zahka@xxxxxxxxx>
> ---
>  drivers/net/ethernet/meta/mpnic/mpnic_pci.c  |  4 ++++
>  drivers/net/ethernet/meta/mpnic/mpnic_txrx.c | 31 ++++++++++++++++++++++++++++
>  drivers/net/ethernet/meta/mpnic/mpnic_txrx.h |  1 +
>  3 files changed, 36 insertions(+)
>
> diff --git a/drivers/net/ethernet/meta/mpnic/mpnic_pci.c b/drivers/net/ethernet/meta/mpnic/mpnic_pci.c
> index 17eae5a9ba16..125fb26edf93 100644
> --- a/drivers/net/ethernet/meta/mpnic/mpnic_pci.c
> +++ b/drivers/net/ethernet/meta/mpnic/mpnic_pci.c
> @@ -11,6 +11,7 @@
>
>  #include "mpnic.h"
>  #include "mpnic_netdev.h"
> +#include "mpnic_txrx.h"
>
>  #define PCI_DEVICE_ID_META_MPNIC       0x0014
>
> @@ -61,6 +62,9 @@ static void mpnic_service_task(struct work_struct *work)
>
>         netdev_lock(netdev);
>
> +       if (netif_carrier_ok(netdev))
> +               mpnic_napi_depletion_check(netdev_priv(netdev));
> +
>         if (netif_running(netdev))
>                 schedule_delayed_work(&mpd->service_task, HZ);
>
> diff --git a/drivers/net/ethernet/meta/mpnic/mpnic_txrx.c b/drivers/net/ethernet/meta/mpnic/mpnic_txrx.c
> index 49061c7316d5..5495e9a9aa65 100644
> --- a/drivers/net/ethernet/meta/mpnic/mpnic_txrx.c
> +++ b/drivers/net/ethernet/meta/mpnic/mpnic_txrx.c
> @@ -55,6 +55,12 @@ static unsigned int mpnic_desc_unused(struct mpnic_ring *ring)
>         return (ring->head - ring->tail - 1) & ring->size_mask;
>  }
>
> +static unsigned int mpnic_desc_used(struct mpnic_ring *ring)
> +{
> +       return (READ_ONCE(ring->tail) - READ_ONCE(ring->head)) &
> +              ring->size_mask;
> +}
> +
>  static struct netdev_queue *mpnic_txring_txq(const struct net_device *dev,
>                                              const struct mpnic_ring *ring)
>  {
> @@ -1393,3 +1399,28 @@ void mpnic_napi_enable(struct mpnic_net *mpn)
>
>         mpnic_wrfl(mpn->mpd);
>  }
> +
> +void mpnic_napi_depletion_check(struct mpnic_net *mpn)
> +{
> +       int i, j, t;
> +
> +       for (i = 0; i < mpn->num_napi; i++) {
> +               struct mpnic_napi_vector *nv = mpn->napi[i];
> +
> +               for (t = nv->txt_count, j = 0; j < nv->rxt_count; j++, t++) {
> +                       /* Check if BDs posted covers a max sized frame
> +                        *  + 1 BD held by RDE as a spare
> +                        *  + 1 BD of extra safety margin
> +                        */

Curious how the "+ 1 BD of extra safety margin" was decided. I
understand the "+ 1 BD held by RDE as a spare" is because of HW
behaviour but wondering about the extra buffer descriptor - if it came
from testing or guarding against something specific.

> +                       if (mpnic_desc_used(&nv->qt[t].sub0) <
> +                           MPNIC_RX_HPQ_DROP_THRS + 2 ||
> +                           mpnic_desc_used(&nv->qt[t].sub1) <
> +                           MPNIC_RX_PPQ_DROP_THRS + 2) {
> +                               mpnic_nv_irq_trigger(nv);
> +                               break;
> +                       }
> +               }
> +       }
> +
> +       mpnic_wrfl(mpn->mpd);
> +}
> diff --git a/drivers/net/ethernet/meta/mpnic/mpnic_txrx.h b/drivers/net/ethernet/meta/mpnic/mpnic_txrx.h
> index 9397010557eb..767d87a36c35 100644
> --- a/drivers/net/ethernet/meta/mpnic/mpnic_txrx.h
> +++ b/drivers/net/ethernet/meta/mpnic/mpnic_txrx.h
> @@ -148,6 +148,7 @@ int mpnic_set_netif_queues(struct mpnic_net *mpn);
>  void mpnic_reset_netif_queues(struct mpnic_net *mpn);
>  void mpnic_napi_enable(struct mpnic_net *mpn);
>  void mpnic_napi_disable(struct mpnic_net *mpn);
> +void mpnic_napi_depletion_check(struct mpnic_net *mpn);
>  void mpnic_enable(struct mpnic_net *mpn);
>  void mpnic_disable(struct mpnic_net *mpn);
>  void mpnic_wait_all_queues_idle(struct mpnic_dev *mpd);
>
> --
> 2.52.0
>
>