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

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Message-ID: <CAABZP2x5H3VQXe3o+Q-ar-o2VO=yLap0WGA0JFWK+U2=rdH53A@mail.gmail.com>
Date:   Thu, 6 Jul 2023 16:13:01 +0800
From:   Zhouyi Zhou <zhouzhouyi@...il.com>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     "fweisbec@...il.com" <fweisbec@...il.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...nel.org" <mingo@...nel.org>,
        "dave@...olabs.net" <dave@...olabs.net>,
        "paulmck@...nel.org" <paulmck@...nel.org>,
        "josh@...htriplett.org" <josh@...htriplett.org>,
        "mpe@...erman.id.au" <mpe@...erman.id.au>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH linux-next][RFC]torture: avoid offline tick_do_timer_cpu

On Thu, Jul 6, 2023 at 3:09 PM Christophe Leroy
<christophe.leroy@...roup.eu> wrote:
>
>
>
> Le 21/11/2022 à 04:51, Zhouyi Zhou a écrit :
> > 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@...il.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);
> >   #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) {
>
> Can you use fonction tick_nohz_full_enabled() instead and avoid the #ifdef ?
Thank Christophe for your wonderful advice, I will follow your advice
next time I prepare a patch.
For this false positive report, 58d766824264 ("tick/nohz: Fix
cpu_is_hotpluggable() by checking with nohz subsystem") has beaten me
in mainline.

Thanks again
Zhouyi
>
> > +                     cpu = (torture_random(&rand) >> 4) % maxcpu;
> > +                     if (cpu >= tick_do_timer_cpu)
> > +                             cpu = (cpu + 1) % (maxcpu + 1);
> > +             } else
> > +#else
> >               cpu = (torture_random(&rand) >> 4) % (maxcpu + 1);
> > +#endif
> >               if (!torture_offline(cpu,
> >                                    &n_offline_attempts, &n_offline_successes,
> >                                    &sum_offline, &min_offline, &max_offline))