netdev - Re: [PATCH] netem: fix 64 bit divide

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Date:   Tue, 14 Nov 2017 09:57:11 -0800
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Stephen Hemminger <stephen@...workplumber.org>,
        davem@...emloft.net, dave.taht@...il.com
Cc:     netdev@...r.kernel.org, Stephen Hemminger <sthemmin@...rosoft.com>
Subject: Re: [PATCH] netem: fix 64 bit divide

On 11/14/2017 09:28 AM, Stephen Hemminger wrote:
> Since times are now expressed in 64 bit nanosecond, need to now do
> true 64 bit divide. Change the name of the function to bette express
> the new units.
> 
> Fixes: 99803171ef04 ("netem: add uapi to express delay and jitter in nanoseconds")
> Reported-by: Randy Dunlap <rdunlap@...radead.org>
> Signed-off-by: Stephen Hemminger <stephen@...workplumber.org>

Hi Stephen,

I still get it. Maybe it's this % operator:

		skb->data[prandom_u32() % skb_headlen(skb)] ^=
			1<<(prandom_u32() % 8);

in net_enqueue().

> ---
>  net/sched/sch_netem.c | 11 ++++-------
>  1 file changed, 4 insertions(+), 7 deletions(-)
> 
> diff --git a/net/sched/sch_netem.c b/net/sched/sch_netem.c
> index b686e755fda9..644323d6081c 100644
> --- a/net/sched/sch_netem.c
> +++ b/net/sched/sch_netem.c
> @@ -339,10 +339,8 @@ static s64 tabledist(s64 mu, s64 sigma,
>  	return  x / NETEM_DIST_SCALE + (sigma / NETEM_DIST_SCALE) * t + mu;
>  }
>  
> -static u64 packet_len_2_sched_time(unsigned int len,
> -				   struct netem_sched_data *q)
> +static u64 packet_time_ns(u64 len, const struct netem_sched_data *q)
>  {
> -	u64 offset;
>  	len += q->packet_overhead;
>  
>  	if (q->cell_size) {
> @@ -352,9 +350,8 @@ static u64 packet_len_2_sched_time(unsigned int len,
>  			cells++;
>  		len = cells * (q->cell_size + q->cell_overhead);
>  	}
> -	offset = (u64)len * NSEC_PER_SEC;
> -	do_div(offset, q->rate);
> -	return offset;
> +
> +	return div64_u64(len * NSEC_PER_SEC, q->rate);
>  }
>  
>  static void tfifo_reset(struct Qdisc *sch)
> @@ -556,7 +553,7 @@ static int netem_enqueue(struct sk_buff *skb, struct Qdisc *sch,
>  				now = last->time_to_send;
>  			}
>  
> -			delay += packet_len_2_sched_time(qdisc_pkt_len(skb), q);
> +			delay += packet_time_ns(qdisc_pkt_len(skb), q);
>  		}
>  
>  		cb->time_to_send = now + delay;
> 


-- 
~Randy