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

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Date:   Thu, 28 Feb 2019 10:10:33 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Leslie Monis' <lesliemonis@...il.com>
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 16:12
> 
> On Wed, Feb 27, 2019 at 10:11:14AM +0000, David Laight wrote:
> > 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
> 
> Hi David,
> 
> You're right, the change does make the result for small
> values different. I made it anyway as the probability
> value is scaled by u64. It is safe to say that q->vars.prob
> holds relatively large values (in its scaled form) in all
> cases where it isn't 0.
> 
> But, I think we can avoid the slow division here. RFC 8033
> does say that using (1 - 1/64) should be sufficient. This
> will give us:
> 	q-vars.prob -= q->vars.prob >> 6;
> which I feel would be much better. What do you reckon?

I think I'd leave it as a division - the compiler should do the shift.
So:
	/* Scale by 98.4% */
	q-vars.prob -= q->vars.prob / 64u;

	David

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