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

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Message-ID: <CA+FuTSerSjYLvK05CgdjFz_oqr6Nn8EKXj-AvuP1L8QqOSC0-A@mail.gmail.com>
Date:	Fri, 6 Feb 2015 17:45:20 -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>,
	Guy Harris <guy@...m.mit.edu>, Dan Collins <dan@...llins.co.nz>
Subject: Re: [PATCH] af_packet: don't pass empty blocks for PACKET_V3

On Thu, Feb 5, 2015 at 10:54 PM, Alexander Drozdov <al.drozdov@...il.com> wrote:
> 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@...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.
>
> I missed something. As pointed by Guy Harris <guy@...m.mit.edu>,
> before the previous patch periodic signal was not delivered. The previous
> patch
> (da413eec729dae5dc by Dan Collins <dan@...llins.co.nz>) is for 3.19 kernel
> only. Should we care about existing 3.19-only applications?

It does sound reasonable to expect processes to handle infinite sleep
on no data if that is the historical behavior of the interface.

>>
>>> 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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