[PATCH] tick/nohz_full: don't abuse smp_call_function_single() in tick_setup_device()

Oleg Nesterov
2024-05-28T12:21:57Z

After the recent commit 5097cbcb38e6 ("sched/isolation: Prevent boot
crash when the boot CPU is nohz_full") the kernel no longer crashes, but
there is another problem.

In this case tick_setup_device() calls tick_take_do_timer_from_boot() to
update tick_do_timer_cpu and this triggers the WARN_ON_ONCE(irqs_disabled)
in smp_call_function_single().

Kill tick_take_do_timer_from_boot() and just use WRITE_ONCE(), the new
comment tries to explain why this is safe (thanks Thomas!).

Fixes: 08ae95f4fd3b ("nohz_full: Allow the boot CPU to be nohz_full")
Link: https://lore.kernel.org/all/20240522151742.GA10400@redhat.com
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
kernel/time/tick-common.c | 39 +++++++++++++--------------------------
1 file changed, 13 insertions(+), 26 deletions(-)

diff --git a/kernel/time/tick-common.c b/kernel/time/tick-common.c
index d88b13076b79..27d0018c8b05 100644
--- a/kernel/time/tick-common.c
+++ b/kernel/time/tick-common.c
@@ -178,26 +178,6 @@ void tick_setup_periodic(struct clock_event_device *dev, int broadcast)
}
}

-#ifdef CONFIG_NO_HZ_FULL
-static void giveup_do_timer(void *info)
-{
- int cpu = *(unsigned int *)info;
-
- WARN_ON(tick_do_timer_cpu != smp_processor_id());
-
- tick_do_timer_cpu = cpu;
-}
-
-static void tick_take_do_timer_from_boot(void)
-{
- int cpu = smp_processor_id();
- int from = tick_do_timer_boot_cpu;
-
- if (from >= 0 && from != cpu)
- smp_call_function_single(from, giveup_do_timer, &cpu, 1);
-}
-#endif
-
/*
* Setup the tick device
*/
@@ -221,19 +201,26 @@ static void tick_setup_device(struct tick_device *td,
tick_next_period = ktime_get();
#ifdef CONFIG_NO_HZ_FULL
/*
- * The boot CPU may be nohz_full, in which case set
- * tick_do_timer_boot_cpu so the first housekeeping
- * secondary that comes up will take do_timer from
- * us.
+ * The boot CPU may be nohz_full, in which case the
+ * first housekeeping secondary will take do_timer()
+ * from us.
*/
if (tick_nohz_full_cpu(cpu))
tick_do_timer_boot_cpu = cpu;

} else if (tick_do_timer_boot_cpu != -1 &&
!tick_nohz_full_cpu(cpu)) {
- tick_take_do_timer_from_boot();
tick_do_timer_boot_cpu = -1;
- WARN_ON(READ_ONCE(tick_do_timer_cpu) != cpu);
+ /*
+ * The boot CPU will stay in periodic (NOHZ disabled)
+ * mode until clocksource_done_booting() called after
+ * smp_init() selects a high resolution clocksource and
+ * timekeeping_notify() kicks the NOHZ stuff alive.
+ *
+ * So this WRITE_ONCE can only race with the READ_ONCE
+ * check in tick_periodic() but this race is harmless.
+ */
+ WRITE_ONCE(tick_do_timer_cpu, cpu);
#endif
}

--
2.25.1.362.g51ebf55