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

Oleg Nesterov
2024-06-10T15:56:50Z

Thomas, et al,

should I redo and/or resend this patch (and the next one on top of it) ?

Or what else can I do to fix this problem?

Thanks,

Oleg.

On 05/28, Oleg Nesterov wrote:
>
> 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
>