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

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Date:	Tue, 10 Feb 2015 04:54:56 -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 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;
> +       }
> +

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);
	u64 val = (u64)ewma_read(&ar->tx_delay_us) *
                   msdu->sk->sk_pacing_rate;

	do_div(val, USEC_PER_SEC);
		
        ACCESS_ONCE(msdu->sk->sk_tx_completion_delay_cushion) =
                    (u32)val;
                 
(WRITE_ONCE() would be better for new kernels, but ACCESS_ONCE() is ok
since we probably want to backport to stable kernels)


Thanks


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