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 <anna-maria@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://lore.kernel.org/r/87cyr49on2.fsf@somnus

diff --git a/kernel/time/timer_migration.c b/kernel/time/timer_migration.c
index e3075e40cb43205037bb94e36976d479870f06ee..ccba875d2234fe582264e7d802dcb62f4864e4f6 100644 (file)
--- 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;