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

[Zhouyi Zhou <zhouzhouyi@gmail.com>]

On Thu, Nov 24, 2022 at 2:49 AM Paul E. McKenney <paulmck@kernel.org> wrote:
>
> On Wed, Nov 23, 2022 at 10:23:11AM +0800, Zhouyi Zhou wrote:
> > On Tue, Nov 22, 2022 at 9:37 AM Paul E. McKenney <paulmck@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>
> > >
> > > Good show chasing this down!
> > Thank Paul for your guidance and encouragement!
> > >
> > > A couple of questions below.
> > The answers 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;
by examine the beginning code of torture_onoff, I see if we has 8
cpus, then maxcpu = 7 (not 8) here,
then cpu is distributed evenly among 0, 1, 2, 3, 4, 5, 6
if we happens to get 6, then cpu+1 below results in 7
> > > > +                     if (cpu >= tick_do_timer_cpu)
> > >
> > > Why is this ">=" instead of "=="?
> > I use probability theory here to let the remaining cpu distribute evenly.
> > Example:
> > we have cpus: 0 1 2 3 4 5 6 7
> > maxcpu = 7
> > tick_do_timer_cpu = 2
> > remaining cpus are: 0 1 3 4 5 6 7
> > if the offline cpu candidate is 2, then the result cpu is 2+1
> > else if the offline cpu candidate is 3, then the result cpu is 3+1
> > ...
> > else if the offline cpu candidate is 6, then the result cpu is 6+1
> > >
> > > > +                             cpu = (cpu + 1) % (maxcpu + 1);
> > we could just use cpu = cpu + 1 here
>
> But won't this get you double the occurrences of CPU 0 compared to the
> other non-tick_do_timer_cpu CPUs?  You might get CPU 0 directly from
> torture_random(), or torture_random() might have given you CPU 7, which
> then wraps to CPU 0.
I think torture_random won't give me CPU 7 as illustrated above,
my code is a little tricky, please correct me if I am wrong.
>
> What am I missing here?
>
> > > > +             } 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?
> > I did a preliminary research about the above question, this is quite
> > complicated for me
> > (because I think I must not bring locks to kernel just because our
> > test frame need them),
>
> Agreed, it would be good to avoid added locks.
>
> > Please give me some days to perform intensive research.
>
> No problem, in fact, please do take the time you need for this.
> As you say, it is not as simple as one might think.
Thanks Paul for your constant encouragement and guidance!
I improved a lot during the process of learning.

Cheers
Zhouyi
>
>                                                         Thanx, Paul
>
> > Thanks again
> > Cheers
> > Zhouyi
> > >
> > >                                                         Thanx, Paul
> > >
> > > >               if (!torture_offline(cpu,
> > > >                                    &n_offline_attempts, &n_offline_successes,
> > > >                                    &sum_offline, &min_offline, &max_offline))
> > > > --
> > > > 2.34.1
> > > >