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sched_ext: Fix vtime delta loss in scx_flatcg cgroup migration

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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: 62addc6dbf ("sched_ext: Use time helpers in BPF schedulers")
Signed-off-by: Wanwu Li <liwanwu@kylinos.cn>
Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
Wanwu Li 2026-08-27 16:07:37 +08:00 committed by Tejun Heo
parent 2376135986
commit b6ee92d7f7
1 changed files with 1 additions and 1 deletions
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2
tools/sched_ext/scx_flatcg.bpf.c
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@ -937,7 +937,7 @@ void BPF_STRUCT_OPS(fcg_cgroup_move, struct task_struct *p,
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);
}