Commit 56534673cea7f ("tick/nohz: Optimize check_tick_dependency() with
early return") added a fast path that returns !val when the tick_stop
tracepoint is disabled.  This is inverted: the slow path returns true
when a dependency IS found (val != 0), but !val returns true when val
is zero (no dependency).  The result is that can_stop_full_tick() sees
"dependency found" when there are none, and the tick never stops on
nohz_full CPUs.

Fix by returning val instead of !val, matching the slow-path semantics.

Fixes: 56534673cea7f ("tick/nohz: Optimize check_tick_dependency() with early return")
Assisted-by: Claude:claude-opus-4-6
Signed-off-by: Josh Snyder <josh@code406.com>
---
 kernel/time/tick-sched.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index f7907fadd63f2..4324f3838ab0d 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -345,7 +345,7 @@ static bool check_tick_dependency(atomic_t *dep)
 	int val = atomic_read(dep);
 
 	if (likely(!tracepoint_enabled(tick_stop)))
-		return !val;
+		return val;
 
 	if (val & TICK_DEP_MASK_POSIX_TIMER) {
 		trace_tick_stop(0, TICK_DEP_MASK_POSIX_TIMER);

---
base-commit: 4f59dbcc2a99c8dd9b438c4e9ccb0e4870ef92f6
change-id: 20260402-fix-idle-tick2-50a4f96df527

Best regards,
--  
Josh

The following commit has been merged into the timers/urgent branch of tip:

Commit-ID:     82b915051d32a68ea3bbe261c93f5620699ff047
Gitweb:        https://git.kernel.org/tip/82b915051d32a68ea3bbe261c93f5620699ff047
Author:        Josh Snyder <josh@code406.com>
AuthorDate:    Thu, 02 Apr 2026 16:23:38 -07:00
Committer:     Thomas Gleixner <tglx@kernel.org>
CommitterDate: Tue, 07 Apr 2026 15:30:21 +02:00

tick/nohz: Fix inverted return value in check_tick_dependency() fast path

Commit 56534673cea7f ("tick/nohz: Optimize check_tick_dependency() with
early return") added a fast path that returns !val when the tick_stop
tracepoint is disabled.

This is inverted: the slow path returns true when a dependency IS found
(val != 0), but !val returns true when val is zero (no dependency).  The
result is that can_stop_full_tick() sees "dependency found" when there are
none, and the tick never stops on nohz_full CPUs.

Fix this by returning !!val instead of !val, matching the slow-path semantics.

Fixes: 56534673cea7f ("tick/nohz: Optimize check_tick_dependency() with early return")
Signed-off-by: Josh Snyder <josh@code406.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Assisted-by: Claude:claude-opus-4-6
Link: https://patch.msgid.link/20260402-fix-idle-tick2-v1-1-eecb589649d3@code406.com
---
 kernel/time/tick-sched.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index f7907fa..36449f0 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -345,7 +345,7 @@ static bool check_tick_dependency(atomic_t *dep)
 	int val = atomic_read(dep);
 
 	if (likely(!tracepoint_enabled(tick_stop)))
-		return !val;
+		return !!val;
 
 	if (val & TICK_DEP_MASK_POSIX_TIMER) {
 		trace_tick_stop(0, TICK_DEP_MASK_POSIX_TIMER);