From: Stephen Hemminger <stephen@networkplumber.org>
To: netdev@vger.kernel.org
Cc: Stephen Hemminger <stephen@networkplumber.org>
Subject: [PATCH 05/12] net/sched: netem: fix probability gaps in 4-state loss model
Date: Fri, 13 Mar 2026 14:15:05 -0700 [thread overview]
Message-ID: <20260313211646.12549-6-stephen@networkplumber.org> (raw)
In-Reply-To: <20260313211646.12549-1-stephen@networkplumber.org>
The 4-state Markov chain in loss_4state() has gaps at the boundaries
between transition probability ranges. The comparisons use:
if (rnd < a4)
else if (a4 < rnd && rnd < a1 + a4)
When rnd equals a boundary value exactly, neither branch matches and
no state transition occurs. The redundant lower-bound check (a4 < rnd)
is already implied by being in the else branch.
Remove the unnecessary lower-bound comparisons so the ranges are
contiguous and every random value produces a transition, matching
the GI (General and Intuitive) loss model specification.
This bug goes back to original implementation of this model.
Fixes: 661b79725fea ("netem: revised correlated loss generator")
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
---
net/sched/sch_netem.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/net/sched/sch_netem.c b/net/sched/sch_netem.c
index 08006a60849e..00ece854280a 100644
--- a/net/sched/sch_netem.c
+++ b/net/sched/sch_netem.c
@@ -237,7 +237,7 @@ static bool loss_4state(struct netem_sched_data *q)
if (rnd < clg->a4) {
clg->state = LOST_IN_GAP_PERIOD;
return true;
- } else if (clg->a4 < rnd && rnd < clg->a1 + clg->a4) {
+ } else if (rnd < clg->a1 + clg->a4) {
clg->state = LOST_IN_BURST_PERIOD;
return true;
} else if (clg->a1 + clg->a4 < rnd) {
@@ -257,7 +257,7 @@ static bool loss_4state(struct netem_sched_data *q)
case LOST_IN_BURST_PERIOD:
if (rnd < clg->a3)
clg->state = TX_IN_BURST_PERIOD;
- else if (clg->a3 < rnd && rnd < clg->a2 + clg->a3) {
+ else if (rnd < clg->a2 + clg->a3) {
clg->state = TX_IN_GAP_PERIOD;
} else if (clg->a2 + clg->a3 < rnd) {
clg->state = LOST_IN_BURST_PERIOD;
--
2.51.0
next prev parent reply other threads:[~2026-03-13 21:17 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-03-13 21:15 [PATCH 00/12] netem: fixes, cleanup, and selftest Stephen Hemminger
2026-03-13 21:15 ` [PATCH 01/12] selftests: net: add netem qdisc test Stephen Hemminger
2026-03-13 21:15 ` [PATCH 02/12] Revert "net/sched: Restrict conditions for adding duplicating netems to qdisc tree" Stephen Hemminger
2026-03-13 21:15 ` [PATCH 03/12] net/sched: netem: add per-CPU recursion guard for duplication Stephen Hemminger
2026-03-14 19:29 ` William Liu
2026-03-15 16:06 ` Stephen Hemminger
2026-03-15 16:19 ` Jamal Hadi Salim
2026-03-15 17:18 ` Stephen Hemminger
2026-03-16 17:52 ` Jamal Hadi Salim
2026-03-13 21:15 ` [PATCH 04/12] net/sched: netem: restructure dequeue to avoid re-entrancy with child qdisc Stephen Hemminger
2026-03-13 21:15 ` Stephen Hemminger [this message]
2026-03-13 21:15 ` [PATCH 06/12] net/sched: netem: fix slot delay calculation overflow Stephen Hemminger
2026-03-13 21:15 ` [PATCH 07/12] net/sched: netem: fix queue limit check to include reordered packets Stephen Hemminger
2026-03-13 21:15 ` [PATCH 08/12] net/sched: netem: null-terminate tfifo linear queue tail Stephen Hemminger
2026-03-13 21:15 ` [PATCH 09/12] net/sched: netem: only reseed PRNG when seed is explicitly provided Stephen Hemminger
2026-03-13 21:15 ` [PATCH 10/12] net/sched: netem: move state enums out of struct netem_sched_data Stephen Hemminger
2026-03-13 21:15 ` [PATCH 11/12] net/sched: netem: remove useless VERSION Stephen Hemminger
2026-03-13 21:15 ` [PATCH 12/12] net/sched: netem: replace pr_info with netlink extack error messages Stephen Hemminger
2026-03-14 14:09 ` [PATCH 00/12] netem: fixes, cleanup, and selftest Jakub Kicinski
2026-03-14 15:39 ` Stephen Hemminger
2026-03-14 15:51 ` Stephen Hemminger
2026-03-14 16:00 ` Jakub Kicinski
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