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sched_ext: Reset dsq_vtime and slice when a task leaves SCX

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When a task leaves the BPF scheduler's control, p->scx.dsq_vtime and
p->scx.slice keep whatever values they last held. The slice value is
core-managed and is refilled on the next enqueue, but dsq_vtime is owned
by the BPF scheduler and is never cleared by the core, so a task that
leaves SCX and later returns carries a stale dsq_vtime across the
round-trip.

The stale values are also visible to other SCX schedulers that inspect
the scx fields of non-SCX tasks.

Fix this by resetting both dsq_vtime and slice in scx_disable_task(),
after ops.disable(), so the BPF scheduler can still observe the task's
final values and non-SCX tasks do not retain stale SCX state.

Signed-off-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
Andrea Righi 2026-06-08 18:55:57 +02:00 committed by Tejun Heo
parent 4cd5de72b6
commit 5771e79e46
1 changed files with 7 additions and 0 deletions
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7
kernel/sched/ext/ext.c
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@ -3647,6 +3647,13 @@ static void scx_disable_task(struct scx_sched *sch, struct task_struct *p)
SCX_CALL_OP_TASK(sch, disable, rq, p);
scx_set_task_state(p, SCX_TASK_READY);
/*
* Reset the SCX-managed fields when @p leaves the BPF scheduler's
* control, after ops.disable() has observed their final values.
*/
p->scx.dsq_vtime = 0;
p->scx.slice = 0;
/*
* Verify the task is not in BPF scheduler's custody. If flag
* transitions are consistent, the flag should always be clear