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

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Date:	Tue, 10 Feb 2015 05:05:13 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Michal Kazior <michal.kazior@...to.com>
Cc:	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 Tue, 2015-02-10 at 04:54 -0800, Eric Dumazet wrote:

> Hi Michal
> 
> This is almost it ;)
> 
> As I said you must do this using u64 arithmetics, we still support 32bit
> kernels.
> 
> Also, >> 20 instead of / 1000000 introduces a 5% error, I would use a
> plain divide, as the compiler will use a reciprocal divide (ie : a
> multiply)
> 
> We use >> 10 instead of /1000 because a 2.4 % error is probably okay.
> 
>         ewma_add(&ar->tx_delay_us,
>                  ktime_to_ns(ktime_sub(ktime_get(),
> skb_cb->stamp)) /
> 	                        NSEC_PER_USEC);

btw I suspect this wont compile on 32 bit kernel

You need to use do_div() as well :

u64 val = ktime_to_ns(ktime_sub(ktime_get(),
				skb_cb->stamp));

do_div(val, NSEC_PER_USEC);

ewma_add(&ar->tx_delay_us, val);


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