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

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Date:	Thu, 12 Feb 2015 00:33:53 -0800
From:	Dave Taht <dave.taht@...il.com>
To:	Michal Kazior <michal.kazior@...to.com>
Cc:	Johannes Berg <johannes@...solutions.net>,
	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 Wed, Feb 11, 2015 at 11:48 PM, Michal Kazior <michal.kazior@...to.com> wrote:
> 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

I will argue strongly in favor of never allowing more than 4ms packets
to accumulate in the firmware.

> 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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Dave Täht

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