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sched_ext: A fix for v6.14-rc4 

pick_task_scx() has a workaround to avoid stalling when the fair class's
 balance() says yes but pick_task() says no. The workaround was incorrectly
 deciding to keep the prev taks running if the task is on SCX even when the
 task is in a sleeping state, which can lead to several confusing failure
 modes. Fix it by testing the prev task is currently queued on SCX instead.
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Merge tag 'sched_ext-for-6.14-rc4-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/sched_ext

Pull sched_ext fix from Tejun Heo:
 "pick_task_scx() has a workaround to avoid stalling when the fair
  class's balance() says yes but pick_task() says no.

  The workaround was incorrectly deciding to keep the prev taks running
  if the task is on SCX even when the task is in a sleeping state, which
  can lead to several confusing failure modes.

  Fix it by testing the prev task is currently queued on SCX instead"

* tag 'sched_ext-for-6.14-rc4-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/sched_ext:
  sched_ext: Fix pick_task_scx() picking non-queued tasks when it's called without balance()
This commit is contained in:
Linus Torvalds 2025-02-26 14:13:11 -08:00
parent 5394eea106 8fef0a3b17
commit e6d3c4e535
1 changed files with 7 additions and 4 deletions
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11
kernel/sched/ext.c
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@ -3117,7 +3117,6 @@ static struct task_struct *pick_task_scx(struct rq *rq)
{
struct task_struct *prev = rq->curr;
struct task_struct *p;
bool prev_on_scx = prev->sched_class == &ext_sched_class;
bool keep_prev = rq->scx.flags & SCX_RQ_BAL_KEEP;
bool kick_idle = false;
@ -3137,14 +3136,18 @@ static struct task_struct *pick_task_scx(struct rq *rq)
* if pick_task_scx() is called without preceding balance_scx().
*/
if (unlikely(rq->scx.flags & SCX_RQ_BAL_PENDING)) {
if (prev_on_scx) {
if (prev->scx.flags & SCX_TASK_QUEUED) {
keep_prev = true;
} else {
keep_prev = false;
kick_idle = true;
}
} else if (unlikely(keep_prev && !prev_on_scx)) {
/* only allowed during transitions */
} else if (unlikely(keep_prev &&
prev->sched_class != &ext_sched_class)) {
/*
* Can happen while enabling as SCX_RQ_BAL_PENDING assertion is
* conditional on scx_enabled() and may have been skipped.
*/
WARN_ON_ONCE(scx_ops_enable_state() == SCX_OPS_ENABLED);
keep_prev = false;
}