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

From: Alexander Drozdov <[email-masked]>
Date: Fri, 06 Feb 2015 09:54:21 +0300

On 05.02.2015 23:01:38 +0300 Willem de Bruijn wrote:
> On Wed, Feb 4, 2015 at 9:58 PM, Alexander Drozdov <[email-masked]> 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 <[email-masked]>,
before the previous patch periodic signal was not delivered. The previous patch
(da413eec729dae5dc by Dan Collins <[email-masked]>) is for 3.19 kernel only.
Should we care about existing 3.19-only applications?
>
>> Signed-off-by: Alexander Drozdov <[email-masked]>
>> ---
>>   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
>>