[PATCH] sched_ext: Fix vtime delta loss in scx_flatcg cgroup migration

[Wanwu Li]

fcg_cgroup_move() lost the signed vtime offset across cgroup
migration in the mechanical conversion to time helpers:
time_delta() clamps negative deltas to 0, so a queued task (whose
dsq_vtime is normally behind the source frontier) loses its
accumulated vtime credit and lands exactly at the destination
frontier instead of keeping its relative position.  Restore the
wrapping signed subtraction.

Fixes: 62addc6dbf36 ("sched_ext: Use time helpers in BPF schedulers")
Signed-off-by: Wanwu Li <liwanwu@xxxxxxxxxx>
---
diff --git a/tools/sched_ext/scx_flatcg.bpf.c b/tools/sched_ext/scx_flatcg.bpf.c
index 64cf4dd964d6..d3186a6038c3 100644
--- a/tools/sched_ext/scx_flatcg.bpf.c
+++ b/tools/sched_ext/scx_flatcg.bpf.c
@@ -931,14 +931,14 @@ void BPF_STRUCT_OPS(fcg_cgroup_move, struct task_struct *p,
 		    struct cgroup *from, struct cgroup *to)
 {
 	struct fcg_cgrp_ctx *from_cgc, *to_cgc;
 	s64 delta;
 
 	/* find_cgrp_ctx() triggers scx_bpf_error() on lookup failures */
 	if (!(from_cgc = find_cgrp_ctx(from)) || !(to_cgc = find_cgrp_ctx(to)))
 		return;
 
-	delta = time_delta(p->scx.dsq_vtime, from_cgc->tvtime_now);
+	delta = (s64)(p->scx.dsq_vtime - from_cgc->tvtime_now);
 	scx_bpf_task_set_dsq_vtime(p, to_cgc->tvtime_now + delta);
 }
 
 s32 BPF_STRUCT_OPS_SLEEPABLE(fcg_init)