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

Frederic Weisbecker
2024-05-30T14:52:44Z

Le Tue, May 28, 2024 at 02:20:19PM +0200, Oleg Nesterov a écrit :
> 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);

Looks good, but can we have a WARN_ONCE(tick_do_timer_cpu != tick_do_timer_boot_cpu)
right before that, just to make sure our assumptions above are right forever and
the boot CPU hasn't stopped the tick up to that point?

And after all, pushing a bit further your subsequent patch, can we get rid of
tick_do_timer_boot_cpu and ifdefery altogether? Such as:

diff --git a/kernel/time/tick-common.c b/kernel/time/tick-common.c
index fb0fdec8719a..63a7bd405de7 100644
--- a/kernel/time/tick-common.c
+++ b/kernel/time/tick-common.c
@@ -48,14 +48,6 @@ ktime_t tick_next_period;
* procedure also covers cpu hotplug.
*/
int tick_do_timer_cpu __read_mostly = TICK_DO_TIMER_BOOT;
-#ifdef CONFIG_NO_HZ_FULL
-/*
- * tick_do_timer_boot_cpu indicates the boot CPU temporarily owns
- * tick_do_timer_cpu and it should be taken over by an eligible secondary
- * when one comes online.
- */
-static int tick_do_timer_boot_cpu __read_mostly = -1;
-#endif

/*
* Debugging: see timer_list.c
@@ -177,26 +169,6 @@ 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
*/
@@ -211,29 +183,28 @@ static void tick_setup_device(struct tick_device *td,
* First device setup ?
*/
if (!td->evtdev) {
+ int timekeeper = READ_ONCE(tick_do_timer_cpu);
/*
* If no cpu took the do_timer update, assign it to
* this cpu:
*/
- if (tick_do_timer_cpu == TICK_DO_TIMER_BOOT) {
+ if (timekeeper == TICK_DO_TIMER_BOOT) {
tick_do_timer_cpu = cpu;
tick_next_period = ktime_get();
-#ifdef CONFIG_NO_HZ_FULL
+ } else if (timekeeper == TICK_DO_TIMER_NONE) {
+ if (WARN_ON_ONCE(tick_nohz_full_enabled()))
+ WRITE_ONCE(tick_do_timer_cpu, cpu);
+ } else if (tick_nohz_full_cpu(timekeeper) && !tick_nohz_full_cpu(cpu)) {
/*
- * 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 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.
*/
- 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(tick_do_timer_cpu != cpu);
-#endif
+ WRITE_ONCE(tick_do_timer_cpu, cpu);
}

/*