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Message-ID: <87cyr49on2.fsf@somnus>
Date: Fri, 05 Apr 2024 10:53:21 +0200
From: Anna-Maria Behnsen <anna-maria@...utronix.de>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: Boqun Feng <boqun.feng@...il.com>, Florian Fainelli
<f.fainelli@...il.com>, Thomas Gleixner <tglx@...utronix.de>, "Russell
King (Oracle)" <linux@...linux.org.uk>, Joel Fernandes
<joel@...lfernandes.org>, Linus Torvalds <torvalds@...ux-foundation.org>,
linux-kernel@...r.kernel.org, kernel-team@...a.com, paulmck@...nel.org,
mingo@...nel.org, rcu@...r.kernel.org, neeraj.upadhyay@....com,
urezki@...il.com, qiang.zhang1211@...il.com, bigeasy@...utronix.de,
chenzhongjin@...wei.com, yangjihong1@...wei.com, rostedt@...dmis.org,
Justin Chen <justin.chen@...adcom.com>
Subject: [PATCH v2] 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@...utronix.de>
Reviewed-by: Frederic Weisbecker <frederic@...nel.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;
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