netdev - Re: [PATCH net-next] sch_netem: Bug fixing in calculating Netem interval

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Date:   Wed, 7 Feb 2018 13:19:34 -0800
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     "Md. Islam" <mislam4@...t.edu>
Cc:     shemminger@...l.org, netdev@...r.kernel.org, edumazet@...gle.com
Subject: Re: [PATCH net-next] sch_netem: Bug fixing in calculating Netem
 interval

On Tue, 6 Feb 2018 23:14:18 -0500
"Md. Islam" <mislam4@...t.edu> wrote:

> In Kernel 4.15.0+, Netem does not work properly.
> 
> Netem setup:
> 
> tc qdisc add dev h1-eth0 root handle 1: netem delay 10ms 2ms
> 
> Result:
> 
> PING 172.16.101.2 (172.16.101.2) 56(84) bytes of data.
> 64 bytes from 172.16.101.2: icmp_seq=1 ttl=64 time=22.8 ms
> 64 bytes from 172.16.101.2: icmp_seq=2 ttl=64 time=10.9 ms
> 64 bytes from 172.16.101.2: icmp_seq=3 ttl=64 time=10.9 ms
> 64 bytes from 172.16.101.2: icmp_seq=5 ttl=64 time=11.4 ms
> 64 bytes from 172.16.101.2: icmp_seq=6 ttl=64 time=11.8 ms
> 64 bytes from 172.16.101.2: icmp_seq=4 ttl=64 time=4303 ms
> 64 bytes from 172.16.101.2: icmp_seq=10 ttl=64 time=11.2 ms
> 64 bytes from 172.16.101.2: icmp_seq=11 ttl=64 time=10.3 ms
> 64 bytes from 172.16.101.2: icmp_seq=7 ttl=64 time=4304 ms
> 64 bytes from 172.16.101.2: icmp_seq=8 ttl=64 time=4303 ms
> 
> Patch:
> 
> diff --git a/net/sched/sch_netem.c b/net/sched/sch_netem.c
> index 7bbc13b..7c179ad 100644
> --- a/net/sched/sch_netem.c
> +++ b/net/sched/sch_netem.c
> @@ -327,7 +327,7 @@ static s64 tabledist(s64 mu, s32 sigma,
> 
>      /* default uniform distribution */
>      if (dist == NULL)
> -        return (rnd % (2 * sigma)) - sigma + mu;
> +        return ((rnd % (2 * sigma)) + mu) - sigma;
> 
>      t = dist->table[rnd % dist->size];
>      x = (sigma % NETEM_DIST_SCALE) * t;
> 
> 
> (rnd % (2 * sigma)) - sigma was overflowing s32. After applying the
> patch, I found following output which is desirable.
> 
> PING 172.16.101.2 (172.16.101.2) 56(84) bytes of data.
> 64 bytes from 172.16.101.2: icmp_seq=1 ttl=64 time=21.1 ms
> 64 bytes from 172.16.101.2: icmp_seq=2 ttl=64 time=8.46 ms
> 64 bytes from 172.16.101.2: icmp_seq=3 ttl=64 time=9.00 ms
> 64 bytes from 172.16.101.2: icmp_seq=4 ttl=64 time=11.8 ms
> 64 bytes from 172.16.101.2: icmp_seq=5 ttl=64 time=8.36 ms
> 64 bytes from 172.16.101.2: icmp_seq=6 ttl=64 time=11.8 ms
> 64 bytes from 172.16.101.2: icmp_seq=7 ttl=64 time=8.11 ms
> 64 bytes from 172.16.101.2: icmp_seq=8 ttl=64 time=10.0 ms
> 64 bytes from 172.16.101.2: icmp_seq=9 ttl=64 time=11.3 ms
> 64 bytes from 172.16.101.2: icmp_seq=10 ttl=64 time=11.5 ms
> 64 bytes from 172.16.101.2: icmp_seq=11 ttl=64 time=10.2 ms

Thanks for reporting the problem.  The original code used 32 bit values
and got converted to 64 bit time values when it went to using ktime.

Your patch fixes the problem, and lets go with it.
The other alternaive would be to make sigma s64 but then it would could
cause unnecessary 64 bit modulus here.

Reviewed-by: Stephen Hemminger <stephen@...workplumber.org>