sched_ext: Fix pick_task_scx() picking non-queued tasks when it's called without...

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>

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 5a81d9a1e31f28f6d90f8a4d450f7e8c0a2a81cd..0f1da199cfc7c7e103d2d01ccd0c818291f01439 100644 (file)
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -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;
        }