Subject: Re: [PATCH linux-next][RFC]torture: avoid offline tick_do_timer_cpu

From: Thomas Gleixner <[email-masked]>
Date: Sat, 26 Nov 2022 18:05:05 +0100

On Mon, Nov 21 2022 at 11:51, Zhouyi Zhou wrote:
> During CPU-hotplug torture (CONFIG_NO_HZ_FULL=y), if we try to
> offline tick_do_timer_cpu, the operation will fail because in
> function tick_nohz_cpu_down:
> ```
> if (tick_nohz_full_running && tick_do_timer_cpu == cpu)
>       return -EBUSY;
> ```
> Above bug was first discovered in torture tests performed in PPC VM

How is this a bug?

> of Open Source Lab of Oregon State University, and reproducable in RISC-V
> and X86-64 (with additional kernel commandline cpu0_hotplug).
>
> In this patch, we avoid offline tick_do_timer_cpu by distribute
> the offlining cpu among remaining cpus.

Please read Documentation/process. Search for 'this patch'...

>
> Signed-off-by: Zhouyi Zhou <[email-masked]>
> ---
>  include/linux/tick.h        |  1 +
>  kernel/time/tick-common.c   |  1 +
>  kernel/time/tick-internal.h |  1 -
>  kernel/torture.c            | 10 ++++++++++
>  4 files changed, 12 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/tick.h b/include/linux/tick.h
> index bfd571f18cfd..23cc0b205853 100644
> --- a/include/linux/tick.h
> +++ b/include/linux/tick.h
> @@ -14,6 +14,7 @@
>  #include <linux/rcupdate.h>
>  
>  #ifdef CONFIG_GENERIC_CLOCKEVENTS
> +extern int tick_do_timer_cpu __read_mostly;
>  extern void __init tick_init(void);
>  /* Should be core only, but ARM BL switcher requires it */
>  extern void tick_suspend_local(void);
> diff --git a/kernel/time/tick-common.c b/kernel/time/tick-common.c
> index 46789356f856..87b9b9afa320 100644
> --- a/kernel/time/tick-common.c
> +++ b/kernel/time/tick-common.c
> @@ -48,6 +48,7 @@ ktime_t tick_next_period;
>   *    procedure also covers cpu hotplug.
>   */
>  int tick_do_timer_cpu __read_mostly = TICK_DO_TIMER_BOOT;
> +EXPORT_SYMBOL_GPL(tick_do_timer_cpu);

No. We are not exporting this just to make a bogus test case happy.

Fix the torture code to handle -EBUSY correctly.

Thanks,

        tglx