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From:   Zhouyi Zhou <zhouzhouyi@gmail.com>
Date:   Thu, 24 Nov 2022 10:18:51 +0800
Message-ID: <CAABZP2yDMTKqkj6r+hsQb=3zdoXOWMQ8Z3QKSxaaVTWv2eS-kw@mail.gmail.com>
Subject: Re: [PATCH linux-next][RFC]torture: avoid offline tick_do_timer_cpu
To:     Frederic Weisbecker <frederic@kernel.org>
Cc:     fweisbec@gmail.com, tglx@linutronix.de, mingo@kernel.org,
        dave@stgolabs.net, paulmck@kernel.org, josh@joshtriplett.org,
        mpe@ellerman.id.au, linuxppc-dev@lists.ozlabs.org,
        linux-kernel@vger.kernel.org
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On Thu, Nov 24, 2022 at 6:37 AM Frederic Weisbecker <frederic@kernel.org> wrote:
>
> 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.
Thank Paul and Frederic both for your guidance!

Things are much easier;-) and I will do it.

Cheers
Zhouyi
>
> Thanks.