On 4/21/26 4:55 AM, Thomas Gleixner wrote:
On Mon, Apr 20 2026 at 23:03, Waiman Long wrote:
In tick_cpu_dying(), if the dying CPU is the current timekeeper,
it has to pass the job over to another CPU. The current code passes
it to another online CPU. However, that CPU may not be a timer tick
housekeeping CPU. If that happens, another CPU will have to manually
take it over again later. Avoid this unnecessary work by directly
assigning an online housekeeping CPU.
Use READ_ONCE/WRITE_ONCE() to access tick_do_timer_cpu in case the
non-HK CPUs may not be in stop machine in the future.
'may not be in the future' is yet more handwaving without
content. Please write your change logs in a way so that people who have
not spent months on this can follow.
@@ -394,12 +395,19 @@ int tick_cpu_dying(unsigned int dying_cpu)
{
/*
* If the current CPU is the timekeeper, it's the only one that can
- * safely hand over its duty. Also all online CPUs are in stop
- * machine, guaranteed not to be idle, therefore there is no
+ * safely hand over its duty. Also all online housekeeping CPUs are
+ * in stop machine, guaranteed not to be idle, therefore there is no
* concurrency and it's safe to pick any online successor.
*/
- if (tick_do_timer_cpu == dying_cpu)
- tick_do_timer_cpu = cpumask_first(cpu_online_mask);
+ if (READ_ONCE(tick_do_timer_cpu) == dying_cpu) {
+ unsigned int new_cpu;
+
+ guard(rcu)();
What's this guard for?
+ new_cpu = cpumask_first_and(cpu_online_mask,
housekeeping_cpumask(HK_TYPE_TICK));
Why has this to use housekeeping_cpumask() and does not use
tick_nohz_full_mask?
The RCU guard is for accessing the HK_TYPE_TICK(HK_TYPE_KERNEL_NOISE)
cpumask. tick_nohz_full_mask cpumask is actually the inverse of
HK_TYPE_TICK cpumask. Yes, I could use cpumask_first_andnot() with
tick_nohz_full_mask. If we make tick_nohz_full_mask an RCU protected
pointer, we still need the guard.
Cheers,
Longman
