netdev - Re: [PATCH net-next v4 5/6] net-timestamp: TCP timestamping

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Date:	Wed, 6 Aug 2014 20:59:30 -0400
From:	Willem de Bruijn <willemb@...gle.com>
To:	David Miller <davem@...emloft.net>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Network Development <netdev@...r.kernel.org>,
	Richard Cochran <richardcochran@...il.com>
Subject: Re: [PATCH net-next v4 5/6] net-timestamp: TCP timestamping

>>> but also that we're going to send multiple reports back to error
>>> queue.
>>
>> I see. The optimization patch to queue timestamps without payload will
>> mitigate that somewhat. I dropped that from the initial patchset, but
>> will fix it up for net-next. It may also be possible to squash
>> multiple timestamped packets on the errqueue together when they all
>> have the same payload, resulting in a single (possibly no-payload)
>> packet with repeating cmsgs IP_RECVERR and SCM_TIMESTAMPING. That
>> would give O(1) overhead regardless of number of retransmits.
>
> We could attach a singly linked list of small sequence number cookies
> to the SKB when it gets queued up.

To avoid queuing a clone of the skb for each timestamp with
sock_queue_err_skb, only queue the first occurrence. Record
subsequent (tstype, tstamp) tuples to the queued skb with
matching tskey, and at ip_recv_error convert each into a cmsg?

> Upon an ACK, we need only look at the entries at the front of the list
> until we see one past the cumulative ACK.
>
> There's a little bit of work to do when SKBs are coalesced or chopped
> up, but in return the timestamp indications the user gets will be very
> accurate.
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