From 7a96a84bfbee96871bb16c70ee3e93d564e190f4 Mon Sep 17 00:00:00 2001 From: Anna-Maria Behnsen Date: Fri, 5 Apr 2024 10:53:21 +0200 Subject: [PATCH] timers/migration: Return early on deactivation 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 Signed-off-by: Thomas Gleixner Reviewed-by: Frederic Weisbecker Link: https://lore.kernel.org/r/87cyr49on2.fsf@somnus --- kernel/time/timer_migration.c | 27 +++++++++++++++++++++++++++ 1 file changed, 27 insertions(+) diff --git a/kernel/time/timer_migration.c b/kernel/time/timer_migration.c index e3075e40cb43..ccba875d2234 100644 --- a/kernel/time/timer_migration.c +++ b/kernel/time/timer_migration.c @@ -751,6 +751,33 @@ bool tmigr_update_events(struct tmigr_group *group, struct tmigr_group *child, 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; -- 2.39.5