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

[Peter Zijlstra]

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()).

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

Sparc64 agains things it should enable preemption when playing dead.

So clearly this isn't going to work well :/

---
 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();
 
 		/*