Re: [PATCH] af_packet: don't pass empty blocks for PACKET_V3

[Alexander Drozdov]

On 05.02.2015 23:01:38 +0300 Willem de Bruijn wrote:
> On Wed, Feb 4, 2015 at 9:58 PM, Alexander Drozdov <al.drozdov@xxxxxxxxx> 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.
I missed something. As pointed by Guy Harris <guy@xxxxxxxxxxxx>,
before the previous patch periodic signal was not delivered. The previous patch
(da413eec729dae5dc by Dan Collins <dan@xxxxxxxxxxxxxx>) is for 3.19 kernel only.
Should we care about existing 3.19-only applications?
> > Signed-off-by: Alexander Drozdov <al.drozdov@xxxxxxxxx>
> > ---
> >   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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