netdev - Re: [PATCH] af_packet: don't pass empty blocks for PACKET

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Date:	Thu, 5 Feb 2015 12:01:38 -0800
From:	Willem de Bruijn <willemb@...gle.com>
To:	Alexander Drozdov <al.drozdov@...il.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Daniel Borkmann <dborkman@...hat.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Network Development <netdev@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] af_packet: don't pass empty blocks for PACKET_V3

On Wed, Feb 4, 2015 at 9:58 PM, Alexander Drozdov <al.drozdov@...il.com> wrote:
> Don't close an empty block on timeout. Its meaningless to
> pass it to the user. Moreover, passing empty blocks wastes
> CPU & buffer space increasing probability of packets
> dropping on small timeouts.
>
> Side effect of this patch is indefinite user-space wait
> in poll on idle links. But, I believe its better to set
> timeout for poll(2) when needed than to get empty blocks
> every millisecond when not needed.

This change would break existing applications that have come
to depend on the periodic signal.

I don't disagree with the argument that the data ready signal
should be sent only when a block is full or a timer expires and
at least some data is waiting, but that is moot at this point.

>
> Signed-off-by: Alexander Drozdov <al.drozdov@...il.com>
> ---
>  net/packet/af_packet.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
> index 9cfe2e1..9a2f70a 100644
> --- a/net/packet/af_packet.c
> +++ b/net/packet/af_packet.c
> @@ -698,6 +698,10 @@ static void prb_retire_rx_blk_timer_expired(unsigned long data)
>
>         if (pkc->last_kactive_blk_num == pkc->kactive_blk_num) {
>                 if (!frozen) {
> +                       if (!BLOCK_NUM_PKTS(pbd)) {
> +                               /* An empty block. Just refresh the timer. */
> +                               goto refresh_timer;
> +                       }
>                         prb_retire_current_block(pkc, po, TP_STATUS_BLK_TMO);
>                         if (!prb_dispatch_next_block(pkc, po))
>                                 goto refresh_timer;
> @@ -798,7 +802,11 @@ static void prb_close_block(struct tpacket_kbdq_core *pkc1,
>                 h1->ts_last_pkt.ts_sec = last_pkt->tp_sec;
>                 h1->ts_last_pkt.ts_nsec = last_pkt->tp_nsec;
>         } else {
> -               /* Ok, we tmo'd - so get the current time */
> +               /* Ok, we tmo'd - so get the current time.
> +                *
> +                * It shouldn't really happen as we don't close empty
> +                * blocks. See prb_retire_rx_blk_timer_expired().
> +                */
>                 struct timespec ts;
>                 getnstimeofday(&ts);
>                 h1->ts_last_pkt.ts_sec = ts.tv_sec;
> --
> 1.9.1
>
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