timer/migration: Fix quick check reporting late expiry

When a CPU is the last active in the hierarchy and it tries to enter
into idle, the quick check looking up the next event towards cpuidle
heuristics may report a too late expiry, such as in the following
scenario:

[GRP1:0]
migrator = NONE
active = NONE
nextevt = T0:0, T0:1
/ \
[GRP0:0] [GRP0:1]
migrator = NONE migrator = NONE
active = NONE active = NONE
nextevt = T0, T1 nextevt = T2
/ \ / \
0 1 2 3
idle idle idle idle

0) The whole system is idle, and CPU 0 was the last migrator. CPU 0 has
a timer (T0), CPU 1 has a timer (T1) and CPU 2 has a timer (T2). The
expire order is T0 < T1 < T2.

[GRP1:0]
migrator = GRP0:0
active = GRP0:0
nextevt = T0:0(i), T0:1
/ \
[GRP0:0] [GRP0:1]
migrator = CPU0 migrator = NONE
active = CPU0 active = NONE
nextevt = T0(i), T1 nextevt = T2
/ \ / \
0 1 2 3
active idle idle idle

1) CPU 0 becomes active. The (i) means a now ignored timer.

[GRP1:0]
migrator = GRP0:0
active = GRP0:0
nextevt = T0:1
/ \
[GRP0:0] [GRP0:1]
migrator = CPU0 migrator = NONE
active = CPU0 active = NONE
nextevt = T1 nextevt = T2
/ \ / \
0 1 2 3
active idle idle idle

2) CPU 0 handles remote. No timer actually expired but ignored timers
have been cleaned out and their sibling's timers haven't been
propagated. As a result the top level's next event is T2 and not T1.

3) CPU 0 tries to enter idle without any global timer enqueued and calls
tmigr_quick_check(). The expiry of T2 is returned instead of the
expiry of T1.

When the quick check returns an expiry that is too late, the cpuidle
governor may pick up a C-state that is too deep. This may be result into
undesired CPU wake up latency if the next timer is actually close enough.

Fix this with assuming that expiries aren't sorted top-down while
performing the quick check. Pick up instead the earliest encountered one
while walking up the hierarchy.

7ee988770326 ("timers: Implement the hierarchical pull model")
Signed-off-by: Frederic Weisbecker <[e-mail]>
Signed-off-by: Thomas Gleixner <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

kernel/time/timer_migration.c

···
  *			  single group active on the way to top level)
  * * nextevt		- when CPU is offline and has to handle timer on his own
  *			  or when on the way to top in every group only a single
- *			  child is active and but @nextevt is before next_expiry
- *			  of top level group
- * * next_expiry (top)	- value of top level group, when on the way to top in
- *			  every group only a single child is active and @nextevt
- *			  is after this value active child.
+ *			  child is active but @nextevt is before the lowest
+ *			  next_expiry encountered while walking up to top level.
+ * * next_expiry	- value of lowest expiry encountered while walking groups
+ *			  if only a single child is active on each and @nextevt
+ *			  is after this lowest expiry.
  */
 u64 tmigr_quick_check(u64 nextevt)
 {
···
 	do {
 		if (!tmigr_check_lonely(group)) {
 			return KTIME_MAX;
-		} else if (!group->parent) {
-			u64 first_global = READ_ONCE(group->next_expiry);
-
-			return min_t(u64, nextevt, first_global);
+		} else {
+			/*
+			 * Since current CPU is active, events may not be sorted
+			 * from bottom to the top because the CPU's event is ignored
+			 * up to the top and its sibling's events not propagated upwards.
+			 * Thus keep track of the lowest observed expiry.
+			 */
+			nextevt = min_t(u64, nextevt, READ_ONCE(group->next_expiry));
+			if (!group->parent)
+				return nextevt;
 		}
 		group = group->parent;
 	} while (group);