psi: Fix race when task wakes up before psi_sched_switch() adjusts flags

When running hackbench in a cgroup with bandwidth throttling enabled,
following PSI splat was observed:

    psi: inconsistent task state! task=1831:hackbench cpu=8 psi_flags=14 clear=0 set=4

When investigating the series of events leading up to the splat,
following sequence was observed:

    [008] d..2.: sched_switch: ... ==> next_comm=hackbench next_pid=1831 next_prio=120
        ...
    [008] dN.2.: dequeue_entity(task delayed): task=hackbench pid=1831 cfs_rq->throttled=0
    [008] dN.2.: pick_task_fair: check_cfs_rq_runtime() throttled cfs_rq on CPU8
    # CPU8 goes into newidle balance and releases the rq lock
        ...
    # CPU15 on same LLC Domain is trying to wakeup hackbench(pid=1831)
    [015] d..4.: psi_flags_change: psi: task state: task=1831:hackbench cpu=8 psi_flags=14 clear=0 set=4 final=14 # Splat (cfs_rq->throttled=1)
    [015] d..4.: sched_wakeup: comm=hackbench pid=1831 prio=120 target_cpu=008 # Task has woken on a throttled hierarchy
    [008] d..2.: sched_switch: prev_comm=hackbench prev_pid=1831 prev_prio=120 prev_state=S ==> ...

psi_dequeue() relies on psi_sched_switch() to set the correct PSI flags
for the blocked entity, however, with the introduction of DELAY_DEQUEUE,
the block task can wakeup when newidle balance drops the runqueue lock
during __schedule().

If a task wakes before psi_sched_switch() adjusts the PSI flags, skip
any modifications in psi_enqueue() which would still see the flags of a
running task and not a blocked one. Instead, rely on psi_sched_switch()
to do the right thing.

Since the status returned by try_to_block_task() may no longer be true
by the time schedule reaches psi_sched_switch(), check if the task is
blocked or not using a combination of task_on_rq_queued() and
p->se.sched_delayed checks.

[ prateek: Commit message, testing, early bailout in psi_enqueue() ]

Fixes: 152e11f ("sched/fair: Implement delayed dequeue") # 1a61510
Signed-off-by: Chengming Zhou <chengming.zhou@linux.dev>
Signed-off-by: K Prateek Nayak <kprateek.nayak@amd.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Chengming Zhou <chengming.zhou@linux.dev>
Link: https://lore.kernel.org/r/20241227061941.2315-1-kprateek.nayak@amd.com

kernel/sched/core.c

@@ -6645,7 +6645,6 @@ static void __sched notrace __schedule(int sched_mode)
 	 * as a preemption by schedule_debug() and RCU.
 	 */
 	bool preempt = sched_mode > SM_NONE;
-	bool block = false;
 	unsigned long *switch_count;
 	unsigned long prev_state;
 	struct rq_flags rf;
@@ -6706,7 +6705,7 @@ static void __sched notrace __schedule(int sched_mode)
 			goto picked;
 		}
 	} else if (!preempt && prev_state) {
-		block = try_to_block_task(rq, prev, prev_state);
+		try_to_block_task(rq, prev, prev_state);
 		switch_count = &prev->nvcsw;
 	}
 
@@ -6752,7 +6751,8 @@ static void __sched notrace __schedule(int sched_mode)
 
 		migrate_disable_switch(rq, prev);
 		psi_account_irqtime(rq, prev, next);
-		psi_sched_switch(prev, next, block);
+		psi_sched_switch(prev, next, !task_on_rq_queued(prev) ||
+					     prev->se.sched_delayed);
 
 		trace_sched_switch(preempt, prev, next, prev_state);
 

kernel/sched/stats.h

@@ -138,6 +138,10 @@ static inline void psi_enqueue(struct task_struct *p, int flags)
 	if (flags & ENQUEUE_RESTORE)
 		return;
 
+	/* psi_sched_switch() will handle the flags */
+	if (task_on_cpu(task_rq(p), p))
+		return;
+
 	if (p->se.sched_delayed) {
 		/* CPU migration of "sleeping" task */
 		SCHED_WARN_ON(!(flags & ENQUEUE_MIGRATED));