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    SubjectRe: [PATCH linux-next][RFC]torture: avoid offline tick_do_timer_cpu
    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@gmail.com>
    > ---
    > 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);

    Please rather make a function for this. This is an internal value
    that we don't want to expose to modules.

    This can be:

    int tick_nohz_full_timekeeper(void)
    {
    if (tick_nohz_full_enabled() && tick_do_timer_cpu >= 0)
    return tick_do_timer_cpu;
    else
    return nr_cpu_ids;
    }

    And then just check if the value is below nr_cpu_ids.

    Thanks.

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