6.13-stable review patch. If anyone has any objections, please let me know.
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From: Tejun Heo tj@kernel.org
commit 8fef0a3b17bb258130a4fcbcb5addf94b25e9ec5 upstream.
a6250aa251ea ("sched_ext: Handle cases where pick_task_scx() is called without preceding balance_scx()") added a workaround to handle the cases where pick_task_scx() is called without prececing balance_scx() which is due to a fair class bug where pick_taks_fair() may return NULL after a true return from balance_fair().
The workaround detects when pick_task_scx() is called without preceding balance_scx() and emulates SCX_RQ_BAL_KEEP and triggers kicking to avoid stalling. Unfortunately, the workaround code was testing whether @prev was on SCX to decide whether to keep the task running. This is incorrect as the task may be on SCX but no longer runnable.
This could lead to a non-runnable task to be returned from pick_task_scx() which cause interesting confusions and failures. e.g. A common failure mode is the task ending up with (!on_rq && on_cpu) state which can cause potential wakers to busy loop, which can easily lead to deadlocks.
Fix it by testing whether @prev has SCX_TASK_QUEUED set. This makes @prev_on_scx only used in one place. Open code the usage and improve the comment while at it.
Signed-off-by: Tejun Heo tj@kernel.org Reported-by: Pat Cody patcody@meta.com Fixes: a6250aa251ea ("sched_ext: Handle cases where pick_task_scx() is called without preceding balance_scx()") Cc: stable@vger.kernel.org # v6.12+ Acked-by: Andrea Righi arighi@nvidia.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- kernel/sched/ext.c | 11 +++++++---- 1 file changed, 7 insertions(+), 4 deletions(-)
--- a/kernel/sched/ext.c +++ b/kernel/sched/ext.c @@ -3097,7 +3097,6 @@ static struct task_struct *pick_task_scx { 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;
@@ -3117,14 +3116,18 @@ static struct task_struct *pick_task_scx * 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; }