Subject: Re: [PATCH net-next] net: sched: pie: fix 64-bit division

From: Leslie Monis <[email-masked]>
Date: Thu, 28 Feb 2019 15:46:25 +0530

On Thu, Feb 28, 2019 at 10:10:33AM +0000, David Laight wrote:
> 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 <[email-masked]>
> > > > ---
> > > >  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
> 
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> Registration No: 1397386 (Wales)
> 

Alright, I'll prepare a patch for this. May I credit you
with a Suggested-by tag?

Thanks,
Leslie