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

From: Zhouyi Zhou <zhouzhouyi@gmail.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: fweisbec@gmail.com, 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
Subject: Re: [PATCH linux-next][RFC]torture: avoid offline tick_do_timer_cpu
Date: Sun, 27 Nov 2022 10:45:34 +0800	[thread overview]
Message-ID: <CAABZP2xNTbrx9iV+KH3VZx1c9Yi97+izNA=XSJQBuOJ4WENFZg@mail.gmail.com> (raw)
In-Reply-To: <87y1rxwsse.ffs@tglx>

Thank Thomas for your guidance

On Sun, Nov 27, 2022 at 1:05 AM Thomas Gleixner <tglx@linutronix.de> wrote:
>
> 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?
Yes, this is a false positive instead.
>
> > 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'...
Documentation/process/submitting-patches.rst says:
"Describe your changes in imperative mood, e.g. "make xyzzy do frotz"
instead of "[This patch] makes xyzzy do frotz" or "[I] changed xyzzy
to do frotz", as if you are giving orders to the codebase to change
its behaviour."

So, I should construct my patch as:
We avoid ... by ...
>
> >
> > 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);
>
> No. We are not exporting this just to make a bogus test case happy.
>
> Fix the torture code to handle -EBUSY correctly.
I am going to do a study on this, for now, I do a grep in the kernel tree:
find . -name "*.c"|xargs grep cpuhp_setup_state|wc -l
The result of the grep command shows that there are 268
cpuhp_setup_state* cases.
which may make our task more complicated.

After my study, should we also take Frederic's proposal as a possible option?
(construct a function for this)
https://lore.kernel.org/lkml/20221123223658.GC1395324@lothringen/

I learned a lot during this process
Many thanks
Zhouyi
>
> Thanks,
>
>         tglx