Re: [PATCH] tick/nohz: Fix cpu_is_hotpluggable() by checking with nohz subsystem

[Zhouyi Zhou]

On Fri, Jan 20, 2023 at 7:53 AM Zhouyi Zhou <zhouzhouyi@xxxxxxxxx> wrote:
>
> On Fri, Jan 20, 2023 at 4:45 AM Joel Fernandes (Google)
> <joel@xxxxxxxxxxxxxxxxx> wrote:
> >
> > For CONFIG_NO_HZ_FULL systems, the tick_do_timer_cpu cannot be offlined.
> > However, cpu_is_hotpluggable() still returns true for those CPUs. This causes
> > torture tests that do offlining to end up trying to offline this CPU causing
> > test failures. Such failure happens on all architectures.
> >
> > Fix it by asking the opinion of the nohz subsystem on whether the CPU can
> > be hotplugged.
> >
> > [ Apply Frederic Weisbecker feedback on refactoring tick_nohz_cpu_down(). ]
> Thanks for your fantastic work
> I applied this fix to linux-5.15.y, and perform new round of rcu
> torture test on PPC VM of Open Source Lab of Oregon State University.
> Could you please wait for the test to finish?
>
> The test results of linux-5.15.y before your patch can be viewed at [1]
> The patched source code of linux-5.15.y can be viewed at [2]
> The ongoing test of patched linux-5.15.y can be viewed at [3]
>
> [1] http://140.211.169.189/linux-stable-rc/tools/testing/selftests/rcutorture/res/2023.01.18-13.22.39-torture/
> [2] http://140.211.169.189/linux-stable-rc/
> [3] http://140.211.169.189/linux-stable-rc/tools/testing/selftests/rcutorture/res/2023.01.19-23.40.55-torture/
The rcutorture test on [3] has finished, by comparing the logs of [1]
and [3], we can see Joel's patch fix the HOTPLUG problem without bring
any new issues.

Tested-by: Zhouyi Zhou <zhouzhouyi@xxxxxxxxx>

Thanks
Zhouyi
>
> Hope to continue to benefit the community.
>
> Thank you all
> Zhouyi
> >
> > Cc: Frederic Weisbecker <frederic@xxxxxxxxxx>
> > Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
> > Cc: Zhouyi Zhou <zhouzhouyi@xxxxxxxxx>
> > Cc: Will Deacon <will@xxxxxxxxxx>
> > Cc: Marc Zyngier <maz@xxxxxxxxxx>
> > Cc: rcu <rcu@xxxxxxxxxxxxxxx>
> > Fixes: 2987557f52b9 ("driver-core/cpu: Expose hotpluggability to the rest of the kernel")
> > Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> > ---
> >  drivers/base/cpu.c       |  3 ++-
> >  include/linux/tick.h     |  2 ++
> >  kernel/time/tick-sched.c | 12 +++++++++++-
> >  3 files changed, 15 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/base/cpu.c b/drivers/base/cpu.c
> > index 55405ebf23ab..450dca235a2f 100644
> > --- a/drivers/base/cpu.c
> > +++ b/drivers/base/cpu.c
> > @@ -487,7 +487,8 @@ static const struct attribute_group *cpu_root_attr_groups[] = {
> >  bool cpu_is_hotpluggable(unsigned int cpu)
> >  {
> >         struct device *dev = get_cpu_device(cpu);
> > -       return dev && container_of(dev, struct cpu, dev)->hotpluggable;
> > +       return dev && container_of(dev, struct cpu, dev)->hotpluggable
> > +               && tick_nohz_cpu_hotpluggable(cpu);
> >  }
> >  EXPORT_SYMBOL_GPL(cpu_is_hotpluggable);
> >
> > diff --git a/include/linux/tick.h b/include/linux/tick.h
> > index bfd571f18cfd..9459fef5b857 100644
> > --- a/include/linux/tick.h
> > +++ b/include/linux/tick.h
> > @@ -216,6 +216,7 @@ extern void tick_nohz_dep_set_signal(struct task_struct *tsk,
> >                                      enum tick_dep_bits bit);
> >  extern void tick_nohz_dep_clear_signal(struct signal_struct *signal,
> >                                        enum tick_dep_bits bit);
> > +extern bool tick_nohz_cpu_hotpluggable(unsigned int cpu);
> >
> >  /*
> >   * The below are tick_nohz_[set,clear]_dep() wrappers that optimize off-cases
> > @@ -280,6 +281,7 @@ static inline void tick_nohz_full_add_cpus_to(struct cpumask *mask) { }
> >
> >  static inline void tick_nohz_dep_set_cpu(int cpu, enum tick_dep_bits bit) { }
> >  static inline void tick_nohz_dep_clear_cpu(int cpu, enum tick_dep_bits bit) { }
> > +static inline bool tick_nohz_cpu_hotpluggable(unsigned int cpu) { return true; }
> >
> >  static inline void tick_dep_set(enum tick_dep_bits bit) { }
> >  static inline void tick_dep_clear(enum tick_dep_bits bit) { }
> > diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> > index 9c6f661fb436..383a060f30c5 100644
> > --- a/kernel/time/tick-sched.c
> > +++ b/kernel/time/tick-sched.c
> > @@ -510,7 +510,7 @@ void __init tick_nohz_full_setup(cpumask_var_t cpumask)
> >         tick_nohz_full_running = true;
> >  }
> >
> > -static int tick_nohz_cpu_down(unsigned int cpu)
> > +static int tick_nohz_cpu_hotplug_ret(unsigned int cpu)
> >  {
> >         /*
> >          * The tick_do_timer_cpu CPU handles housekeeping duty (unbound
> > @@ -522,6 +522,16 @@ static int tick_nohz_cpu_down(unsigned int cpu)
> >         return 0;
> >  }
> >
> > +static int tick_nohz_cpu_down(unsigned int cpu)
> > +{
> > +       return tick_nohz_cpu_hotplug_ret(cpu);
> > +}
> > +
> > +bool tick_nohz_cpu_hotpluggable(unsigned int cpu)
> > +{
> > +       return tick_nohz_cpu_hotplug_ret(cpu) == 0;
> > +}
> > +
> >  void __init tick_nohz_init(void)
> >  {
> >         int cpu, ret;
> > --
> > 2.39.0.246.g2a6d74b583-goog
> >