sched_ext: Call wakeup_preempt() in local_dsq_post_enq()

There are several edge cases (see linked thread) where an IMMED task
can be left lingering on a local DSQ if an RT task swoops in at the
wrong time. All of these edge cases are due to rq->next_class being idle
even after dispatching a task to rq's local DSQ. We should bump
rq->next_class to &ext_sched_class as soon as we've inserted a task into
the local DSQ.

To optimize the common case of rq->next_class == &ext_sched_class,
only call wakeup_preempt() if rq->next_class is below EXT. If next_class
is EXT or above, wakeup_preempt() is a no-op anyway.

This lets us also simplify the preempt_curr() logic a bit since
wakeup_preempt() will call preempt_curr() for us if next_class is
below EXT.

Link: https://lore.kernel.org/all/DHZPHUFXB4N3.2RY28MUEWBNYK@google.com/
Signed-off-by: Kuba Piecuch <jpiecuch@google.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 9eda20e5fdb8a3c55a31a24e5e612fba494fd945..cac0b18239fea7b8a4e0283a3abfeeed51a72301 100644 (file)
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -1402,14 +1402,51 @@ static void local_dsq_post_enq(struct scx_sched *sch, struct scx_dispatch_q *dsq
                               struct task_struct *p, u64 enq_flags)
 {
        struct rq *rq = container_of(dsq, struct rq, scx.local_dsq);
-       bool preempt = false;
 
        call_task_dequeue(sch, rq, p, 0);
 
+       /*
+        * Note that @rq's lock may be dropped between this enqueue and @p
+        * actually getting on CPU. This gives higher-class tasks (e.g. RT)
+        * an opportunity to wake up on @rq and prevent @p from running.
+        * Here are some concrete examples:
+        *
+        * Example 1:
+        *
+        * We dispatch two tasks from a single ops.dispatch():
+        * - First, a local task to this CPU's local DSQ;
+        * - Second, a local/remote task to a remote CPU's local DSQ.
+        * We must drop the local rq lock in order to finish the second
+        * dispatch. In that time, an RT task can wake up on the local rq.
+        *
+        * Example 2:
+        *
+        * We dispatch a local/remote task to a remote CPU's local DSQ.
+        * We must drop the remote rq lock before the dispatched task can run,
+        * which gives an RT task an opportunity to wake up on the remote rq.
+        *
+        * Both examples work the same if we replace dispatching with moving
+        * the tasks from a user-created DSQ.
+        *
+        * We must detect these wakeups so that we can re-enqueue IMMED tasks
+        * from @rq's local DSQ. scx_wakeup_preempt() serves exactly this
+        * purpose, but for it to be invoked, we must ensure that we bump
+        * @rq->next_class to &ext_sched_class if it's currently idle.
+        *
+        * wakeup_preempt() does the bumping, and since we only invoke it if
+        * @rq->next_class is below &ext_sched_class, it will also
+        * resched_curr(rq).
+        */
+       if (sched_class_above(p->sched_class, rq->next_class))
+               wakeup_preempt(rq, p, 0);
+
        /*
         * If @rq is in balance, the CPU is already vacant and looking for the
         * next task to run. No need to preempt or trigger resched after moving
         * @p into its local DSQ.
+        * Note that the wakeup_preempt() above may have already triggered
+        * a resched if @rq->next_class was idle. It's harmless, since
+        * need_resched is cleared immediately after task pick.
         */
        if (rq->scx.flags & SCX_RQ_IN_BALANCE)
                return;
@@ -1417,11 +1454,8 @@ static void local_dsq_post_enq(struct scx_sched *sch, struct scx_dispatch_q *dsq
        if ((enq_flags & SCX_ENQ_PREEMPT) && p != rq->curr &&
            rq->curr->sched_class == &ext_sched_class) {
                rq->curr->scx.slice = 0;
-               preempt = true;
-       }
-
-       if (preempt || sched_class_above(&ext_sched_class, rq->curr->sched_class))
                resched_curr(rq);
+       }
 }
 
 static void dispatch_enqueue(struct scx_sched *sch, struct rq *rq,