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

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Message-ID: <CA+FuTSdnuBw4ex7Hj=ko_r0P7SqPs5gA2qcTX1tHMbo3YndMJQ@mail.gmail.com>
Date:	Wed, 6 Aug 2014 15:20:39 -0400
From:	Willem de Bruijn <willemb@...gle.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Network Development <netdev@...r.kernel.org>,
	David Miller <davem@...emloft.net>,
	Richard Cochran <richardcochran@...il.com>
Subject: Re: [PATCH net-next v4 5/6] net-timestamp: TCP timestamping

On Wed, Aug 6, 2014 at 10:37 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Wed, 2014-08-06 at 10:23 -0400, Willem de Bruijn wrote:
>> > I believe you missed fact that a GSO packet can be split into 2 or many
>> > skbs, when tcp_write_xmit() has to send a lower amount.
>> >
>> > This is done in tso_fragment()
>>
>> Thanks, I missed that case. I'll look into it.
>>
>> > Also, I am a bit worried about retransmits ?
>>
>> For the same reason, i.e., possibly calling tcp_fragment?
>
> Yes,

Good catch, thanks. Both cases only affect the correctness of this
feature, not existing code, so I will work on a separate fix for net
later.

> 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.

I just sent the fix to the other issues.

>
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