From 90f19b2816f5d243a8daf36ca83d9d9d04f00e4c Mon Sep 17 00:00:00 2001 From: Tejun Heo Date: Sat, 5 Sep 2026 06:09:56 -1000 Subject: [PATCH] sched_ext: Use @prev's scheduler for the keep decisions in dispatch_one() dispatch_one() tests ops flags and bypass state against the root scheduler in both places where it decides to keep running @prev: the early keep of a @prev with slice left tests the root's bypass state, and the keep-last at the end tests the root's SCX_OPS_ENQ_LAST and bypass state. Both are properties of the scheduler @prev belongs to, and put_prev_task_scx(), which acts on the outcome, reads them from that scheduler. When @prev belongs to a sub-scheduler the two sides disagree. The keep-last case is visible. The root set SCX_OPS_ENQ_LAST, so a lone @prev of a sub-scheduler is not kept and is enqueued with SCX_ENQ_LAST to a sub-scheduler that never opted in. This trips the WARN_ON_ONCE in put_prev_task_scx() for the missing flag, and the sub-scheduler queues the task like any other and triggers no follow-up scheduling event, which can lead to stalls. Test SCX_OPS_ENQ_LAST and bypass state on @prev's sched in both places and charge SCX_EV_DISPATCH_KEEP_LAST to it. Read the sched at each decision, as the dispatch in between can drop the rq lock. Fixes: 88234b075c3f ("sched_ext: Introduce scx_task_sched[_rcu]()") Signed-off-by: Tejun Heo Reviewed-by: Andrea Righi --- kernel/sched/ext/ext.c | 23 ++++++++++++++--------- 1 file changed, 14 insertions(+), 9 deletions(-) diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c index 120540cdda74..adf5993fa597 100644 --- a/kernel/sched/ext/ext.c +++ b/kernel/sched/ext/ext.c @@ -2955,7 +2955,7 @@ static enum scx_dsp_verdict dispatch_one(struct rq *rq, struct task_struct *prev * test. */ if ((prev->scx.flags & SCX_TASK_QUEUED) && prev->scx.slice && - !scx_bypassing(root_sch, cpu)) { + !scx_bypassing(scx_task_sched(prev), cpu)) { verdict = SCX_DSP_PREV; goto has_tasks; } @@ -2972,15 +2972,20 @@ static enum scx_dsp_verdict dispatch_one(struct rq *rq, struct task_struct *prev goto has_tasks; /* - * Didn't find another task to run. Keep running @prev unless - * %SCX_OPS_ENQ_LAST is in effect. + * Didn't find another task to run. Keep running @prev unless its own + * scheduler set %SCX_OPS_ENQ_LAST and takes the enqueue instead, see + * put_prev_task_scx(). Read the scheduler here as the dispatch above + * may have dropped the rq lock while @prev changed class or scheduler. */ - if ((prev->scx.flags & SCX_TASK_QUEUED) && - (!(root_sch->ops.flags & SCX_OPS_ENQ_LAST) || scx_bypassing(root_sch, cpu)) && - scx_task_can_stay_on_cpu(rq, prev)) { - __scx_add_event(root_sch, SCX_EV_DISPATCH_KEEP_LAST, 1); - verdict = SCX_DSP_PREV; - goto has_tasks; + if (prev->scx.flags & SCX_TASK_QUEUED) { + struct scx_sched *prev_sch = scx_task_sched(prev); + + if ((!(prev_sch->ops.flags & SCX_OPS_ENQ_LAST) || + scx_bypassing(prev_sch, cpu)) && scx_task_can_stay_on_cpu(rq, prev)) { + __scx_add_event(prev_sch, SCX_EV_DISPATCH_KEEP_LAST, 1); + verdict = SCX_DSP_PREV; + goto has_tasks; + } } rq->scx.flags &= ~SCX_RQ_IN_DISPATCH; return SCX_DSP_NONE;