netdev - Re: [PATCH net-next v2 2/3] bnxt: count packets discarded because of netpoll

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Date:   Wed, 25 Aug 2021 23:58:10 -0700
From:   Michael Chan <michael.chan@...adcom.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     David Miller <davem@...emloft.net>,
        Netdev <netdev@...r.kernel.org>, olteanv@...il.com
Subject: Re: [PATCH net-next v2 2/3] bnxt: count packets discarded because of netpoll

On Wed, Aug 25, 2021 at 6:47 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> bnxt may discard packets if Rx completions are consumed
> in an attempt to let netpoll make progress. It should be
> exteremely rare in practice but nonetheless such events
> should be counted.
>
> Since completion ring memory is allocated dynamically use
> a similar scheme to what is done for HW stats to save them.
>
> Report the stats in rx_dropped and per-netdev ethtool
> counter. Chances that users care which ring dropped are
> very low.
>
> Signed-off-by: Jakub Kicinski <kuba@...nel.org>
> ---
>  drivers/net/ethernet/broadcom/bnxt/bnxt.c     | 31 ++++++++++++++-----
>  drivers/net/ethernet/broadcom/bnxt/bnxt.h     |  2 ++
>  .../net/ethernet/broadcom/bnxt/bnxt_ethtool.c |  6 ++++
>  3 files changed, 32 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/net/ethernet/broadcom/bnxt/bnxt.c b/drivers/net/ethernet/broadcom/bnxt/bnxt.c
> index d39449e7b236..d12a9052388f 100644
> --- a/drivers/net/ethernet/broadcom/bnxt/bnxt.c
> +++ b/drivers/net/ethernet/broadcom/bnxt/bnxt.c
> @@ -2003,6 +2003,7 @@ static int bnxt_force_rx_discard(struct bnxt *bp,
>         struct rx_cmp *rxcmp;
>         u16 cp_cons;
>         u8 cmp_type;
> +       int ret;
>
>         cp_cons = RING_CMP(tmp_raw_cons);
>         rxcmp = (struct rx_cmp *)
> @@ -2031,7 +2032,10 @@ static int bnxt_force_rx_discard(struct bnxt *bp,
>                 tpa_end1->rx_tpa_end_cmp_errors_v2 |=
>                         cpu_to_le32(RX_TPA_END_CMP_ERRORS);
>         }
> -       return bnxt_rx_pkt(bp, cpr, raw_cons, event);
> +       ret = bnxt_rx_pkt(bp, cpr, raw_cons, event);
> +       if (ret != -EBUSY)
> +               cpr->sw_stats.rx.rx_netpoll_discards += 1;

To be more accurate, we should also skip counting if ret is 0.  0
means it is a TPA_START completion for the start of an aggregated
packet.  We haven't dropped this aggregated packet yet until we
process the TPA_END completion.

> +       return ret;
>  }
>
>  u32 bnxt_fw_health_readl(struct bnxt *bp, int reg_idx)

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