linux-kernel - Re: [PATCH net v2] ixgbe: fix double clean of tx descriptors with xdp

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Date:   Thu, 22 Aug 2019 09:23:46 -0700
From:   William Tu <u9012063@...il.com>
To:     Ilya Maximets <i.maximets@...sung.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Björn Töpel <bjorn.topel@...el.com>,
        Magnus Karlsson <magnus.karlsson@...el.com>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
        intel-wired-lan <intel-wired-lan@...ts.osuosl.org>,
        Eelco Chaudron <echaudro@...hat.com>,
        Alexander Duyck <alexander.duyck@...il.com>
Subject: Re: [PATCH net v2] ixgbe: fix double clean of tx descriptors with xdp

On Thu, Aug 22, 2019 at 5:30 AM Ilya Maximets <i.maximets@...sung.com> wrote:
>
> Tx code doesn't clear the descriptors' status after cleaning.
> So, if the budget is larger than number of used elems in a ring, some
> descriptors will be accounted twice and xsk_umem_complete_tx will move
> prod_tail far beyond the prod_head breaking the comletion queue ring.

s/comletion/completion/

>
> Fix that by limiting the number of descriptors to clean by the number
> of used descriptors in the tx ring.
>
> 'ixgbe_clean_xdp_tx_irq()' function refactored to look more like
> 'ixgbe_xsk_clean_tx_ring()' since we don't need most of the
> complications implemented in the regular 'ixgbe_clean_tx_irq()'
> and we're allowed to directly use 'next_to_clean' and 'next_to_use'
> indexes.
>
> Fixes: 8221c5eba8c1 ("ixgbe: add AF_XDP zero-copy Tx support")
> Signed-off-by: Ilya Maximets <i.maximets@...sung.com>
> ---

Tested-by: William Tu <u9012063@...il.com>

Instead of measuring tx performance at the tx machine, I measured the TX
performance at the other side (the traffic generating machine).  This time it
is more consistent and showing not much difference with (5.9Mpps) and
without this patch (6.1Mpps).

>
> Version 2:
>   * 'ixgbe_clean_xdp_tx_irq()' refactored to look more like
>     'ixgbe_xsk_clean_tx_ring()'.
>
>  drivers/net/ethernet/intel/ixgbe/ixgbe_xsk.c | 34 ++++++++------------
>  1 file changed, 13 insertions(+), 21 deletions(-)
>

<snip>