[PATCH 2/2] sched_ext/scx_flatcg: make cgv_node_less() wraparound-safe

[Tao Cui]

From: Tao Cui <cuitao@xxxxxxxxxx>

cgv_node_less() compares cvtimes with a plain <, which breaks once
cvtime wraps. A weight-1 cgroup in a hierarchy summing to 10000
advances cvtime at up to 10000x wall time, so 2^64 ns of cvtime is
weeks of continuous saturation away -- unlikely but reachable on a
long-running host. At the wrap instant the plain comparison puts the
wrapped node behind everything else permanently.

Compare with (s64)(a - b) < 0 instead, as CFS does for vruntime. A
cyclic comparison is valid as an rbtree comparator only because
cgrp_cap_budget() clamps every node to within max_budget behind
cvtime_now, so any two nodes are far less than 2^63 apart and the
cyclic order agrees with the true order.

Compile-tested and smoke-tested in a VM: weight distribution and
dispatch unaffected.

Fixes: 7b742aa2c2c9 ("sched_ext: Add a cgroup scheduler which uses flattened hierarchy")
Reported-by: Sashiko <sashiko-bot@xxxxxxxxxx>
Link: https://lore.kernel.org/r/3f1ce004-e259-4e72-a5f7-14a5050053bd@xxxxxxxxx
Signed-off-by: Tao Cui <cuitao@xxxxxxxxxx>
---
 tools/sched_ext/scx_flatcg.bpf.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/tools/sched_ext/scx_flatcg.bpf.c b/tools/sched_ext/scx_flatcg.bpf.c
index 454ebb820c5e..be03b409db5e 100644
--- a/tools/sched_ext/scx_flatcg.bpf.c
+++ b/tools/sched_ext/scx_flatcg.bpf.c
@@ -144,7 +144,8 @@ static bool cgv_node_less(struct bpf_rb_node *a, const struct bpf_rb_node *b)
 	cgc_a = container_of(a, struct cgv_node, rb_node);
 	cgc_b = container_of(b, struct cgv_node, rb_node);
 
-	return cgc_a->cvtime < cgc_b->cvtime;
+	/* wrap-safe: cap_budget keeps nodes within 2^63 of each other */
+	return (s64)(cgc_a->cvtime - cgc_b->cvtime) < 0;
 }
 
 static struct fcg_cpu_ctx *find_cpu_ctx(void)
-- 
2.43.0