[PATCH v2] timers/migration: Return early on deactivation

Anna-Maria Behnsen
2024-04-05T08:53:24Z

Commit 4b6f4c5a67c0 ("timer/migration: Remove buggy early return on
deactivation") removed the logic to return early in tmigr_update_events()
on deactivation. With this the problem with a not properly updated first
global event in a hierarchy containing only a single group was fixed.

But when having a look at this code path with a hierarchy with more than a
single level, now unnecessary work is done (example is partially copied
from the message of the commit mentioned above):

[GRP1:0]
migrator = GRP0:0
active = GRP0:0
nextevt = T0:0i, T0:1
/ \
[GRP0:0] [GRP0:1]
migrator = 0 migrator = NONE
active = 0 active = NONE
nextevt = T0i, T1 nextevt = T2
/ \ / \
0 (T0i) 1 (T1) 2 (T2) 3
active idle idle idle

0) CPU 0 is active thus its event is ignored (the letter 'i') and so are
upper levels' events. CPU 1 is idle and has the timer T1 enqueued.
CPU 2 also has a timer. The expiry order is T0 (ignored) < T1 < T2

[GRP1:0]
migrator = GRP0:0
active = GRP0:0
nextevt = T0:0i, T0:1
/ \
[GRP0:0] [GRP0:1]
migrator = NONE migrator = NONE
active = NONE active = NONE
nextevt = T1 nextevt = T2
/ \ / \
0 (T0i) 1 (T1) 2 (T2) 3
idle idle idle idle

1) CPU 0 goes idle without global event queued. Therefore KTIME_MAX is
pushed as its next expiry and its own event kept as "ignore". Without this
early return the following steps happen in tmigr_update_events() when
child = null and group = GRP0:0 :

lock(GRP0:0->lock);
timerqueue_del(GRP0:0, T0i);
unlock(GRP0:0->lock);

[GRP1:0]
migrator = NONE
active = NONE
nextevt = T0:0, T0:1
/ \
[GRP0:0] [GRP0:1]
migrator = NONE migrator = NONE
active = NONE active = NONE
nextevt = T1 nextevt = T2
/ \ / \
0 (T0i) 1 (T1) 2 (T2) 3
idle idle idle idle

2) The change now propagates up to the top. Then tmigr_update_events()
updates the group event of GRP0:0 and executes the following steps
(child = GRP0:0 and group = GRP0:0):

lock(GRP0:0->lock);
lock(GRP1:0->lock);
evt = tmigr_next_groupevt(GRP0:0); -> this removes the ignored events
in GRP0:0
... update GRP1:0 group event and timerqueue ...
unlock(GRP1:0->lock);
unlock(GRP0:0->lock);

So the dance in 1) with locking the GRP0:0->lock and removing the T0i from
the timerqueue is redundand as this is done nevertheless in 2) when
tmigr_next_groupevt(GRP0:0) is executed.

Revert commit 4b6f4c5a67c0 ("timer/migration: Remove buggy early return on
deactivation") and add a condition into return path to skip the return
only, when hierarchy contains a single group. Adapt comments accordingly.

Fixes: 4b6f4c5a67c0 ("timer/migration: Remove buggy early return on deactivation")
Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
---
kernel/time/timer_migration.c | 27 +++++++++++++++++++++++++++
1 file changed, 27 insertions(+)

--- a/kernel/time/timer_migration.c
+++ b/kernel/time/timer_migration.c
@@ -751,6 +751,33 @@ bool tmigr_update_events(struct tmigr_gr

first_childevt = evt = data->evt;

+ /*
+ * Walking the hierarchy is required in any case when a
+ * remote expiry was done before. This ensures to not lose
+ * already queued events in non active groups (see section
+ * "Required event and timerqueue update after a remote
+ * expiry" in the documentation at the top).
+ *
+ * The two call sites which are executed without a remote expiry
+ * before, are not prevented from propagating changes through
+ * the hierarchy by the return:
+ * - When entering this path by tmigr_new_timer(), @evt->ignore
+ * is never set.
+ * - tmigr_inactive_up() takes care of the propagation by
+ * itself and ignores the return value. But an immediate
+ * return is possible if there is a parent, sparing group
+ * locking at this level, because the upper walking call to
+ * the parent will take care about removing this event from
+ * within the group and update next_expiry accordingly.
+ *
+ * However if there is no parent, ie: the hierarchy has only a
+ * single level so @group is the top level group, make sure the
+ * first event information of the group is updated properly and
+ * also handled properly, so skip this fast return path.
+ */
+ if (evt->ignore && !remote && group->parent)
+ return true;
+
raw_spin_lock(&group->lock);

childstate.state = 0;