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

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Date:	Thu, 12 Feb 2015 08:48:24 +0100
From:	Michal Kazior <michal.kazior@...to.com>
To:	Johannes Berg <johannes@...solutions.net>
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 11 February 2015 at 09:57, Michal Kazior <michal.kazior@...to.com> wrote:
> On 10 February 2015 at 15:19, Johannes Berg <johannes@...solutions.net> wrote:
>> On Tue, 2015-02-10 at 11:33 +0100, Michal Kazior wrote:
>>
>>> +       if (msdu->sk) {
>>> +               ewma_add(&ar->tx_delay_us,
>>> +                        ktime_to_ns(ktime_sub(ktime_get(), skb_cb->stamp)) /
>>> +                        NSEC_PER_USEC);
>>> +
>>> +               ACCESS_ONCE(msdu->sk->sk_tx_completion_delay_cushion) =
>>> +                               (ewma_read(&ar->tx_delay_us) *
>>> +                                msdu->sk->sk_pacing_rate) >> 20;
>>> +       }
>>
>> To some extent, every wifi driver is going to have this problem. Perhaps
>> we should do this in mac80211?
>
> 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).

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?


Michał
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