Merge tag 'timers-core-2025-11-30' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull timer core updates from Thomas Gleixner:

 - Prevent a thundering herd problem when the timekeeper CPU is delayed
   and a large number of CPUs compete to acquire jiffies_lock to do the
   update. Limit it to one CPU with a separate "uncontended" atomic
   variable.

 - A set of improvements for the timer migration mechanism:

     - Support imbalanced NUMA trees correctly

     - Support dynamic exclusion of CPUs from the migrator duty to allow
       the cpuset/isolation mechanism to exclude them from handling
       timers of remote idle CPUs

 - The usual small updates, cleanups and enhancements

* tag 'timers-core-2025-11-30' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  timers/migration: Exclude isolated cpus from hierarchy
  cpumask: Add initialiser to use cleanup helpers
  sched/isolation: Force housekeeping if isolcpus and nohz_full don't leave any
  cgroup/cpuset: Rename update_unbound_workqueue_cpumask() to update_isolation_cpumasks()
  timers/migration: Use scoped_guard on available flag set/clear
  timers/migration: Add mask for CPUs available in the hierarchy
  timers/migration: Rename 'online' bit to 'available'
  selftests/timers/nanosleep: Add tests for return of remaining time
  selftests/timers: Clean up kernel version check in posix_timers
  time: Fix a few typos in time[r] related code comments
  time: tick-oneshot: Add missing Return and parameter descriptions to kernel-doc
  hrtimer: Store time as ktime_t in restart block
  timers/migration: Remove dead code handling idle CPU checking for remote timers
  timers/migration: Remove unused "cpu" parameter from tmigr_get_group()
  timers/migration: Assert that hotplug preparing CPU is part of stable active hierarchy
  timers/migration: Fix imbalanced NUMA trees
  timers/migration: Remove locking on group connection
  timers/migration: Convert "while" loops to use "for"
  tick/sched: Limit non-timekeeper CPUs calling jiffies update

1 files changed, 26 insertions, 4 deletions

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 466e083c8272..8ddf74e705d3 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -201,6 +201,27 @@ static inline void tick_sched_flag_clear(struct tick_sched *ts,
 	ts->flags &= ~flag;
 }
 
+/*
+ * Allow only one non-timekeeper CPU at a time update jiffies from
+ * the timer tick.
+ *
+ * Returns true if update was run.
+ */
+static bool tick_limited_update_jiffies64(struct tick_sched *ts, ktime_t now)
+{
+	static atomic_t in_progress;
+	int inp;
+
+	inp = atomic_read(&in_progress);
+	if (inp || !atomic_try_cmpxchg(&in_progress, &inp, 1))
+		return false;
+
+	if (ts->last_tick_jiffies == jiffies)
+		tick_do_update_jiffies64(now);
+	atomic_set(&in_progress, 0);
+	return true;
+}
+
 #define MAX_STALLED_JIFFIES 5
 
 static void tick_sched_do_timer(struct tick_sched *ts, ktime_t now)
@@ -239,10 +260,11 @@ static void tick_sched_do_timer(struct tick_sched *ts, ktime_t now)
 		ts->stalled_jiffies = 0;
 		ts->last_tick_jiffies = READ_ONCE(jiffies);
 	} else {
-		if (++ts->stalled_jiffies == MAX_STALLED_JIFFIES) {
-			tick_do_update_jiffies64(now);
-			ts->stalled_jiffies = 0;
-			ts->last_tick_jiffies = READ_ONCE(jiffies);
+		if (++ts->stalled_jiffies >= MAX_STALLED_JIFFIES) {
+			if (tick_limited_update_jiffies64(ts, now)) {
+				ts->stalled_jiffies = 0;
+				ts->last_tick_jiffies = READ_ONCE(jiffies);
+			}
 		}
 	}