netdev - RE: [PATCH net-next] net: sched: pie: fix 64-bit division

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Message-ID: <44c1c447befd45e49a06b1e3d3f5f7a6@AcuMS.aculab.com>
Date:   Wed, 27 Feb 2019 10:11:14 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Leslie Monis' <lesliemonis@...il.com>,
        "davem@...emloft.net" <davem@...emloft.net>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: RE: [PATCH net-next] net: sched: pie: fix 64-bit division

From: Leslie Monis
> Sent: 27 February 2019 01:00
> Use div_u64() to resolve build failures on 32-bit platforms.
> 
> Fixes: 3f7ae5f3dc52 ("net: sched: pie: add more cases to auto-tune alpha and beta")
> Signed-off-by: Leslie Monis <lesliemonis@...il.com>
> ---
>  net/sched/sch_pie.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/sched/sch_pie.c b/net/sched/sch_pie.c
> index 4c0670b6aec1..f93cfe034c72 100644
> --- a/net/sched/sch_pie.c
> +++ b/net/sched/sch_pie.c
> @@ -429,7 +429,7 @@ static void calculate_probability(struct Qdisc *sch)
>  	 */
> 
>  	if (qdelay == 0 && qdelay_old == 0 && update_prob)
> -		q->vars.prob = (q->vars.prob * 98) / 100;
> +		q->vars.prob = 98 * div_u64(q->vars.prob, 100);

This has significantly different rounding after the change.
The result for small values is very different.
The alterative:
		q->vars.prob -= div_u64(q->vars.prob, 50);
is much nearer to the original - but never goes to zero.

If the 98% decay factor isn't critical then you could remove
1/64th or 1/32nd + 1/16th to avoid the slow division.

	David

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