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From:   Sasha Levin <Alexander.Levin@microsoft.com>
To:     "stable@vger.kernel.org" <stable@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
CC:     "Md. Islam" <mislam4@kent.edu>,
        "David S . Miller" <davem@davemloft.net>,
        Sasha Levin <Alexander.Levin@microsoft.com>
Subject: [PATCH AUTOSEL for 4.15 165/189] sch_netem: Bug fixing in calculating
 Netem interval
Thread-Topic: [PATCH AUTOSEL for 4.15 165/189] sch_netem: Bug fixing in
 calculating Netem interval
Thread-Index: AQHTz5hb7x4hUgs8LkePNx6+3sNUTg==
Date:   Mon, 9 Apr 2018 00:19:00 +0000
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From: "Md. Islam" <mislam4@kent.edu>

[ Upstream commit 043e337f555e610ad8237fd23522d97c968d72b9 ]

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=3D1 ttl=3D64 time=3D22.8 ms
64 bytes from 172.16.101.2: icmp_seq=3D2 ttl=3D64 time=3D10.9 ms
64 bytes from 172.16.101.2: icmp_seq=3D3 ttl=3D64 time=3D10.9 ms
64 bytes from 172.16.101.2: icmp_seq=3D5 ttl=3D64 time=3D11.4 ms
64 bytes from 172.16.101.2: icmp_seq=3D6 ttl=3D64 time=3D11.8 ms
64 bytes from 172.16.101.2: icmp_seq=3D4 ttl=3D64 time=3D4303 ms
64 bytes from 172.16.101.2: icmp_seq=3D10 ttl=3D64 time=3D11.2 ms
64 bytes from 172.16.101.2: icmp_seq=3D11 ttl=3D64 time=3D10.3 ms
64 bytes from 172.16.101.2: icmp_seq=3D7 ttl=3D64 time=3D4304 ms
64 bytes from 172.16.101.2: icmp_seq=3D8 ttl=3D64 time=3D4303 ms

Patch:

(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=3D1 ttl=3D64 time=3D21.1 ms
64 bytes from 172.16.101.2: icmp_seq=3D2 ttl=3D64 time=3D8.46 ms
64 bytes from 172.16.101.2: icmp_seq=3D3 ttl=3D64 time=3D9.00 ms
64 bytes from 172.16.101.2: icmp_seq=3D4 ttl=3D64 time=3D11.8 ms
64 bytes from 172.16.101.2: icmp_seq=3D5 ttl=3D64 time=3D8.36 ms
64 bytes from 172.16.101.2: icmp_seq=3D6 ttl=3D64 time=3D11.8 ms
64 bytes from 172.16.101.2: icmp_seq=3D7 ttl=3D64 time=3D8.11 ms
64 bytes from 172.16.101.2: icmp_seq=3D8 ttl=3D64 time=3D10.0 ms
64 bytes from 172.16.101.2: icmp_seq=3D9 ttl=3D64 time=3D11.3 ms
64 bytes from 172.16.101.2: icmp_seq=3D10 ttl=3D64 time=3D11.5 ms
64 bytes from 172.16.101.2: icmp_seq=3D11 ttl=3D64 time=3D10.2 ms

Reviewed-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
---
 net/sched/sch_netem.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/sched/sch_netem.c b/net/sched/sch_netem.c
index 2aeca57f9bd0..73c4bea821cb 100644
--- a/net/sched/sch_netem.c
+++ b/net/sched/sch_netem.c
@@ -327,7 +327,7 @@ static s64 tabledist(s64 mu, s32 sigma,
=20
 	/* default uniform distribution */
 	if (dist =3D=3D NULL)
-		return (rnd % (2 * sigma)) - sigma + mu;
+		return ((rnd % (2 * sigma)) + mu) - sigma;
=20
 	t =3D dist->table[rnd % dist->size];
 	x =3D (sigma % NETEM_DIST_SCALE) * t;
--=20
2.15.1