netdev - Re: [PATCH v3 net-next 08/14] mlx4: use order-0 pages for RX

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Date:   Mon, 13 Mar 2017 11:20:00 +0200
From:   Saeed Mahameed <saeedm@....mellanox.co.il>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Alexander Duyck <alexander.duyck@...il.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Alexander Duyck <alexander.h.duyck@...el.com>,
        "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Tariq Toukan <tariqt@...lanox.com>,
        Saeed Mahameed <saeedm@...lanox.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Brenden Blanco <bblanco@...mgrid.com>,
        Alexei Starovoitov <ast@...nel.org>
Subject: Re: [PATCH v3 net-next 08/14] mlx4: use order-0 pages for RX

On Sun, Mar 12, 2017 at 6:49 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Sun, 2017-03-12 at 17:49 +0200, Saeed Mahameed wrote:
>> On Sun, Mar 12, 2017 at 5:29 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>> > On Sun, 2017-03-12 at 07:57 -0700, Eric Dumazet wrote:
>> >
>> >> Problem is XDP TX :
>> >>
>> >> I do not see any guarantee mlx4_en_recycle_tx_desc() runs while the NAPI
>> >> RX is owned by current cpu.
>> >>
>> >> Since TX completion is using a different NAPI, I really do not believe
>> >> we can avoid an atomic operation, like a spinlock, to protect the list
>> >> of pages ( ring->page_cache )
>> >
>> > A quick fix for net-next would be :
>> >
>>
>> Hi Eric, Good catch.
>>
>> I don't think we need to complicate with an expensive spinlock,
>>  we can simply fix this by not enabling interrupts on XDP TX CQ (not
>> arm this CQ at all).
>> and handle XDP TX CQ completion from the RX NAPI context, in a serial
>> (Atomic) manner before handling RX completions themselves.
>> This way locking is not required since all page cache handling is done
>> from the same context (RX NAPI).
>>
>> This is how we do this in mlx5, and this is the best approach
>> (performance wise) since we dealy XDP TX CQ completions handling
>> until we really need the space they hold (On new RX packets).
>
> SGTM, can you provide the patch for mlx4 ?
>

of course, We will send it soon.

> Thanks !
>
>