netdev - Re: Throughput regression with `tcp: refine TSO autosizing`

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Date:	Tue, 24 Feb 2015 11:24:55 +0100
From:	Johannes Berg <johannes@...solutions.net>
To:	Michal Kazior <michal.kazior@...to.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Neal Cardwell <ncardwell@...gle.com>,
	linux-wireless <linux-wireless@...r.kernel.org>,
	Network Development <netdev@...r.kernel.org>,
	Eyal Perry <eyalpe@....mellanox.co.il>
Subject: Re: Throughput regression with `tcp: refine TSO autosizing`

On Thu, 2015-02-12 at 08:48 +0100, Michal Kazior wrote:

> > Good point. I was actually thinking about it. I can try cooking a
> > patch unless you want to do it yourself :-)
> 
> I've taken a look into this. The most obvious place to add the
> timestamp for each packet would be ieee80211_tx_info (i.e. the
> skb->cb[48]). The problem is it's very tight there. Even squeezing 2
> bytes (allowing up to 64ms of tx completion delay which I'm worried
> won't be enough) will be troublesome. Some drivers already use every
> last byte of their allowance on 64bit archs (e.g. ar5523 uses entire
> 40 bytes of driver_data).

Couldn't we just repurpose the existing skb->tstamp field for this, as
long as the skb is fully contained within the wireless layer?

Actually, it looks like we can't, since I guess timestamping options can
be turned on on any socket.

> I wonder if it's okay to bump skb->cb to 56 bytes to avoid the cascade
> of changes required to implement the tx completion delay accounting?

I have no doubt that would be rejected :)

johannes

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