netdev - Re: [PATCH] net-packet: tx timestamping on tpacket ring

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Date:	Sat, 13 Apr 2013 20:00:25 -0400
From:	Willem de Bruijn <willemb@...gle.com>
To:	Daniel Borkmann <dborkman@...hat.com>
Cc:	Paul Chavent <paul.chavent@...ra.fr>,
	Richard Cochran <richardcochran@...il.com>,
	Eric Dumazet <edumazet@...gle.com>,
	daniel.borkmann@....ee.ethz.ch, xemul@...allels.com,
	ebiederm@...ssion.com, netdev@...r.kernel.org
Subject: Re: [PATCH] net-packet: tx timestamping on tpacket ring

On Sat, Apr 13, 2013 at 6:18 PM, Daniel Borkmann <dborkman@...hat.com> wrote:
> On 04/13/2013 08:56 PM, Willem de Bruijn wrote:
>>
>> When transmit timestamping is enabled at the socket level, have
>> writes to a PACKET_TX_RING record a timestamp for the generated
>> skbuffs. Tx timestamps are always looped to the application over
>> the socket error queue.
>
>
> Nitpick: if so, then this should go to net-next (subject line).

Thanks.
>
>> The patch also loops software timestamps back into the ring.
>>
>> Signed-off-by: Willem de Bruijn <willemb@...gle.com>
>> ---
>>   net/core/skbuff.c      |  2 +-
>>   net/packet/af_packet.c | 42 ++++++++++++++++++++++++++++++++++++++++++
>>   2 files changed, 43 insertions(+), 1 deletion(-)
>
> [...]
>
>> +static void __packet_set_timestamp(struct packet_sock *po, void *frame,
>> +                                  ktime_t tstamp)
>> +{
>> +       struct tpacket_hdr *h1;
>> +       struct tpacket2_hdr *h2;
>> +       struct timespec ts;
>> +
>> +       if (!tstamp.tv64 || !sock_flag(&po->sk,
>> SOCK_TIMESTAMPING_SOFTWARE))
>> +               return;
>> +
>> +       ts = ktime_to_timespec(tstamp);
>> +
>> +       switch (po->tp_version) {
>> +       case TPACKET_V1:
>> +               h1 = frame;
>> +               h1->tp_sec = ts.tv_sec;
>> +               h1->tp_usec = ts.tv_nsec / NSEC_PER_USEC;
>> +
>> +               flush_dcache_page(pgv_to_page(&h1->tp_sec));
>> +               flush_dcache_page(pgv_to_page(&h1->tp_usec));
>
>
> Hmm, not sure, but could we also flush the dcache only once?
>
>
>> +               break;
>> +       case TPACKET_V2:
>> +               h2 = frame;
>> +               h2->tp_sec = ts.tv_sec;
>> +               h2->tp_nsec = ts.tv_nsec;
>> +
>> +               flush_dcache_page(pgv_to_page(&h2->tp_sec));
>> +               flush_dcache_page(pgv_to_page(&h2->tp_nsec));
>> +               break;
>> +       case TPACKET_V3:
>> +       default:
>> +               WARN(1, "TPACKET version not supported.\n");
>> +               BUG();
>> +       }
>> +
>> +
>
>
> Nitpick: one space too much.
>
>
>> +       smp_wmb();
>> +}
>> +
>>   static void *packet_lookup_frame(struct packet_sock *po,
>>                 struct packet_ring_buffer *rb,
>>                 unsigned int position,
>> @@ -1900,6 +1939,7 @@ static void tpacket_destruct_skb(struct sk_buff
>> *skb)
>>                 ph = skb_shinfo(skb)->destructor_arg;
>>                 BUG_ON(atomic_read(&po->tx_ring.pending) == 0);
>>                 atomic_dec(&po->tx_ring.pending);
>> +               __packet_set_timestamp(po, ph, skb->tstamp);
>>                 __packet_set_status(po, ph, TP_STATUS_AVAILABLE);
>>         }
>>
>> @@ -2119,6 +2159,8 @@ static int tpacket_snd(struct packet_sock *po,
>> struct msghdr *msg)
>>                         }
>>                 }
>>
>> +               sock_tx_timestamp(&po->sk, &skb_shinfo(skb)->tx_flags);
>> +
>
>
> Hmm, so in case nobody wants to use timestamping on TX (which might be the
> majority
> of people), we have to go through those 3 additional if statements in
> sock_tx_timestamp()
> each time? Shouldn't we rather make the TX_RING faster? ;-)

A static key, similar to netstamp_needed, might make it cheaper, but
may be more complexity than warranted for a corner case. Simpler is
testing only the software timestamp, and in tpacket_fill_skb, where
the sk struct is already accessed. I don't feel strongly about merging
given the performance trade-off: just wanted to see if it worked.
Happy to resubmit either revision (or a better idea).

>
>>                 skb->destructor = tpacket_destruct_skb;
>>                 __packet_set_status(po, ph, TP_STATUS_SENDING);
>>                 atomic_inc(&po->tx_ring.pending);
>>
>
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