Re: [PATCH HACK RFC] cpu: Prevent late-arriving interrupts from disrupting offline

["Paul E. McKenney" <paulmck@linux.ibm.com>]

On Mon, Jun 03, 2019 at 10:38:48AM +0200, Peter Zijlstra wrote:
> On Sat, Jun 01, 2019 at 06:12:53PM -0700, Paul E. McKenney wrote:
> > Scheduling-clock interrupts can arrive late in the CPU-offline process,
> > after idle entry and the subsequent call to cpuhp_report_idle_dead().
> > Once execution passes the call to rcu_report_dead(), RCU is ignoring
> > the CPU, which results in lockdep complaints when the interrupt handler
> > uses RCU:
> 
> > diff --git a/kernel/cpu.c b/kernel/cpu.c
> > index 448efc06bb2d..3b33d83b793d 100644
> > --- a/kernel/cpu.c
> > +++ b/kernel/cpu.c
> > @@ -930,6 +930,7 @@ void cpuhp_report_idle_dead(void)
> >  	struct cpuhp_cpu_state *st = this_cpu_ptr(&cpuhp_state);
> >  
> >  	BUG_ON(st->state != CPUHP_AP_OFFLINE);
> > +	local_irq_disable();
> >  	rcu_report_dead(smp_processor_id());
> >  	st->state = CPUHP_AP_IDLE_DEAD;
> >  	udelay(1000);
> 
> Urgh... I'd almost suggest we do something like the below.
> 
> 
> But then I started looking at the various arch_cpu_idle_dead()
> implementations and ran into arm's implementation, which is calling
> complete() where generic code already established this isn't possible
> (see for example cpuhp_report_idle_dead()).

Yeah, my patch that would have changed that never was acked or taken
by the maintainer, as discussed later in this thread.

> And then there's powerpc which for some obscure reason thinks it needs
> to enable preemption when dying ?! pseries_cpu_die() actually calls
> msleep() ?!?!

Isn't pseries_cpu_die() invoked via the smp_ops->cpu_die() function
pointer, whch is invoked from __cpu_die() in arch/powerpc/kernel/smp.c?
Then, if I am reading the code correctly, __cpu_die() is invoked from
takedown_cpu(), which is invoked not from the dying CPU but rather from
a surviving CPU.  Or am I misreading the code?

> Sparc64 agains things it should enable preemption when playing dead.
> 
> So clearly this isn't going to work well :/

Well, it looks like it will work at least as well as my patch.  I will
test it out this evening, ten timezones east of my usual location.  ;-)

							Thanx, Paul

> ---
>  include/linux/tick.h | 10 ----------
>  kernel/sched/idle.c  |  5 +++--
>  2 files changed, 3 insertions(+), 12 deletions(-)
> 
> diff --git a/include/linux/tick.h b/include/linux/tick.h
> index f92a10b5e112..196a0a7bfc4f 100644
> --- a/include/linux/tick.h
> +++ b/include/linux/tick.h
> @@ -134,14 +134,6 @@ extern unsigned long tick_nohz_get_idle_calls(void);
>  extern unsigned long tick_nohz_get_idle_calls_cpu(int cpu);
>  extern u64 get_cpu_idle_time_us(int cpu, u64 *last_update_time);
>  extern u64 get_cpu_iowait_time_us(int cpu, u64 *last_update_time);
> -
> -static inline void tick_nohz_idle_stop_tick_protected(void)
> -{
> -	local_irq_disable();
> -	tick_nohz_idle_stop_tick();
> -	local_irq_enable();
> -}
> -
>  #else /* !CONFIG_NO_HZ_COMMON */
>  #define tick_nohz_enabled (0)
>  static inline int tick_nohz_tick_stopped(void) { return 0; }
> @@ -164,8 +156,6 @@ static inline ktime_t tick_nohz_get_sleep_length(ktime_t *delta_next)
>  }
>  static inline u64 get_cpu_idle_time_us(int cpu, u64 *unused) { return -1; }
>  static inline u64 get_cpu_iowait_time_us(int cpu, u64 *unused) { return -1; }
> -
> -static inline void tick_nohz_idle_stop_tick_protected(void) { }
>  #endif /* !CONFIG_NO_HZ_COMMON */
>  
>  #ifdef CONFIG_NO_HZ_FULL
> diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
> index 80940939b733..e4bc4aa739b8 100644
> --- a/kernel/sched/idle.c
> +++ b/kernel/sched/idle.c
> @@ -241,13 +241,14 @@ static void do_idle(void)
>  		check_pgt_cache();
>  		rmb();
>  
> +		local_irq_disable();
> +
>  		if (cpu_is_offline(cpu)) {
> -			tick_nohz_idle_stop_tick_protected();
> +			tick_nohz_idle_stop_tick();
>  			cpuhp_report_idle_dead();
>  			arch_cpu_idle_dead();
>  		}
>  
> -		local_irq_disable();
>  		arch_cpu_idle_enter();
>  
>  		/*
>