[PATCH v2] sched/fair: Forfeit vruntime on yield

[Fernand Sieber <sieberf@amazon.com>]

If a task yields, the scheduler may decide to pick it again. The task in
turn may decide to yield immediately or shortly after, leading to a tight
loop of yields.

If there's another runnable task as this point, the deadline will be
increased by the slice at each loop. This can cause the deadline to runaway
pretty quickly, and subsequent elevated run delays later on as the task
doesn't get picked again. The reason the scheduler can pick the same task
again and again despite its deadline increasing is because it may be the
only eligible task at that point.

Fix this by making the task forfeiting its remaining vruntime and pushing
the deadline one slice ahead. This implements yield behavior more
authentically.

Fixes: 147f3efaa24182 ("sched/fair: Implement an EEVDF-like scheduling  policy")
Link: https://lore.kernel.org/r/20250401123622.584018-1-sieberf@amazon.com
Link: https://lore.kernel.org/r/20250911095113.203439-1-sieberf@amazon.com
Signed-off-by: Fernand Sieber <sieberf@amazon.com>

Changes in v2:
- Implement vruntime forfeiting approach suggested by Peter Zijlstra
- Updated commit name
- Previous Reviewed-by tags removed due to algorithm change
---
 kernel/sched/fair.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 7a14da5396fb..cc4ef7213d43 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9036,6 +9036,7 @@ static void yield_task_fair(struct rq *rq)
 	 */
 	rq_clock_skip_update(rq);

+	se->vruntime = se->deadline;
 	se->deadline += calc_delta_fair(se->slice, se);
 }

--
2.34.1




Amazon Development Centre (South Africa) (Proprietary) Limited
29 Gogosoa Street, Observatory, Cape Town, Western Cape, 7925, South Africa
Registration Number: 2004 / 034463 / 07