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

[Paul E. McKenney]

On Mon, Nov 21, 2022 at 11:51:40AM +0800, 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
> 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.
> 
> Signed-off-by: Zhouyi Zhou <zhouzhouyi@xxxxxxxxx>

Good show chasing this down!

A couple of questions below.

> ---
>  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);
>  #ifdef CONFIG_NO_HZ_FULL
>  /*
>   * tick_do_timer_boot_cpu indicates the boot CPU temporarily owns
> diff --git a/kernel/time/tick-internal.h b/kernel/time/tick-internal.h
> index 649f2b48e8f0..8953dca10fdd 100644
> --- a/kernel/time/tick-internal.h
> +++ b/kernel/time/tick-internal.h
> @@ -15,7 +15,6 @@
>  
>  DECLARE_PER_CPU(struct tick_device, tick_cpu_device);
>  extern ktime_t tick_next_period;
> -extern int tick_do_timer_cpu __read_mostly;
>  
>  extern void tick_setup_periodic(struct clock_event_device *dev, int broadcast);
>  extern void tick_handle_periodic(struct clock_event_device *dev);
> diff --git a/kernel/torture.c b/kernel/torture.c
> index 789aeb0e1159..bccbdd33dda2 100644
> --- a/kernel/torture.c
> +++ b/kernel/torture.c
> @@ -33,6 +33,7 @@
>  #include <linux/delay.h>
>  #include <linux/stat.h>
>  #include <linux/slab.h>
> +#include <linux/tick.h>
>  #include <linux/trace_clock.h>
>  #include <linux/ktime.h>
>  #include <asm/byteorder.h>
> @@ -358,7 +359,16 @@ torture_onoff(void *arg)
>  			schedule_timeout_interruptible(HZ / 10);
>  			continue;
>  		}
> +#ifdef CONFIG_NO_HZ_FULL
> +		/* do not offline tick do timer cpu */
> +		if (tick_nohz_full_running) {
> +			cpu = (torture_random(&rand) >> 4) % maxcpu;
> +			if (cpu >= tick_do_timer_cpu)

Why is this ">=" instead of "=="?

> +				cpu = (cpu + 1) % (maxcpu + 1);
> +		} else
> +#else
>  		cpu = (torture_random(&rand) >> 4) % (maxcpu + 1);
> +#endif

What happens if the value of tick_do_timer_cpu changes between the time of
the check above and the call to torture_offline() below?  Alternatively,
how is such a change in value prevented?

							Thanx, Paul

>  		if (!torture_offline(cpu,
>  				     &n_offline_attempts, &n_offline_successes,
>  				     &sum_offline, &min_offline, &max_offline))
> -- 
> 2.34.1
>