Re: [PATCH 2/4] cpuidle: Fix missing need_resched() check after rcu_idle_enter()

[Dmitry Osipenko]

05.01.2021 20:25, Dmitry Osipenko пишет:
> 22.12.2020 04:37, Frederic Weisbecker пишет:
>> Entering RCU idle mode may cause a deferred wake up of an RCU NOCB_GP
>> kthread (rcuog) to be serviced.
>>
>> Usually a wake up happening while running the idle task is spotted in
>> one of the need_resched() checks carefully placed within the idle loop
>> that can break to the scheduler.
>>
>> Unfortunately within cpuidle the call to rcu_idle_enter() is already
>> beyond the last generic need_resched() check. Some drivers may perform
>> their own checks like with mwait_idle_with_hints() but many others don't
>> and we may halt the CPU with a resched request unhandled, leaving the
>> task hanging.
>>
>> Fix this with performing a last minute need_resched() check after
>> calling rcu_idle_enter().
>>
>> Reported-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
>> Fixes: 1098582a0f6c (sched,idle,rcu: Push rcu_idle deeper into the idle path)
>> Cc: stable@xxxxxxxxxxxxxxx
>> Cc: Daniel Lezcano <daniel.lezcano@xxxxxxxxxx>
>> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
>> Cc: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
>> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
>> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
>> Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
>> ---
>>  drivers/cpuidle/cpuidle.c | 33 +++++++++++++++++++++++++--------
>>  1 file changed, 25 insertions(+), 8 deletions(-)
>>
>> diff --git a/drivers/cpuidle/cpuidle.c b/drivers/cpuidle/cpuidle.c
>> index ef2ea1b12cd8..4cc1ba49ce05 100644
>> --- a/drivers/cpuidle/cpuidle.c
>> +++ b/drivers/cpuidle/cpuidle.c
>> @@ -134,8 +134,8 @@ int cpuidle_find_deepest_state(struct cpuidle_driver *drv,
>>  }
>>  
>>  #ifdef CONFIG_SUSPEND
>> -static void enter_s2idle_proper(struct cpuidle_driver *drv,
>> -				struct cpuidle_device *dev, int index)
>> +static int enter_s2idle_proper(struct cpuidle_driver *drv,
>> +			       struct cpuidle_device *dev, int index)
>>  {
>>  	ktime_t time_start, time_end;
>>  	struct cpuidle_state *target_state = &drv->states[index];
>> @@ -151,7 +151,14 @@ static void enter_s2idle_proper(struct cpuidle_driver *drv,
>>  	stop_critical_timings();
>>  	if (!(target_state->flags & CPUIDLE_FLAG_RCU_IDLE))
>>  		rcu_idle_enter();
>> -	target_state->enter_s2idle(dev, drv, index);
>> +	/*
>> +	 * Last need_resched() check must come after rcu_idle_enter()
>> +	 * which may wake up RCU internal tasks.
>> +	 */
>> +	if (!need_resched())
>> +		target_state->enter_s2idle(dev, drv, index);
>> +	else
>> +		index = -EBUSY;
>>  	if (WARN_ON_ONCE(!irqs_disabled()))
>>  		local_irq_disable();
>>  	if (!(target_state->flags & CPUIDLE_FLAG_RCU_IDLE))
>> @@ -159,10 +166,13 @@ static void enter_s2idle_proper(struct cpuidle_driver *drv,
>>  	tick_unfreeze();
>>  	start_critical_timings();
>>  
>> -	time_end = ns_to_ktime(local_clock());
>> +	if (index > 0) {
> 
> index=0 is valid too
> 
>> +		time_end = ns_to_ktime(local_clock());
>> +		dev->states_usage[index].s2idle_time += ktime_us_delta(time_end, time_start);
>> +		dev->states_usage[index].s2idle_usage++;
>> +	}
>>  
>> -	dev->states_usage[index].s2idle_time += ktime_us_delta(time_end, time_start);
>> -	dev->states_usage[index].s2idle_usage++;
>> +	return index;
>>  }
>>  
>>  /**
>> @@ -184,7 +194,7 @@ int cpuidle_enter_s2idle(struct cpuidle_driver *drv, struct cpuidle_device *dev)
>>  	 */
>>  	index = find_deepest_state(drv, dev, U64_MAX, 0, true);
>>  	if (index > 0) {
>> -		enter_s2idle_proper(drv, dev, index);
>> +		index = enter_s2idle_proper(drv, dev, index);
>>  		local_irq_enable();
>>  	}
>>  	return index;
>> @@ -234,7 +244,14 @@ int cpuidle_enter_state(struct cpuidle_device *dev, struct cpuidle_driver *drv,
>>  	stop_critical_timings();
>>  	if (!(target_state->flags & CPUIDLE_FLAG_RCU_IDLE))
>>  		rcu_idle_enter();
>> -	entered_state = target_state->enter(dev, drv, index);
>> +	/*
>> +	 * Last need_resched() check must come after rcu_idle_enter()
>> +	 * which may wake up RCU internal tasks.
>> +	 */
>> +	if (!need_resched())
>> +		entered_state = target_state->enter(dev, drv, index);
>> +	else
>> +		entered_state = -EBUSY;
>>  	if (!(target_state->flags & CPUIDLE_FLAG_RCU_IDLE))
>>  		rcu_idle_exit();
>>  	start_critical_timings();
>>
> 
> This patch causes a hardlock on NVIDIA Tegra using today's linux-next.
> Disabling coupled idling state helps. Please fix thanks in advance.
> 

This isn't a proper fix, but it works:

diff --git a/drivers/cpuidle/cpuidle-tegra.c
b/drivers/cpuidle/cpuidle-tegra.c
index 191966dc8d02..ecc5d9b31553 100644
--- a/drivers/cpuidle/cpuidle-tegra.c
+++ b/drivers/cpuidle/cpuidle-tegra.c
@@ -148,7 +148,7 @@ static int tegra_cpuidle_c7_enter(void)

 static int tegra_cpuidle_coupled_barrier(struct cpuidle_device *dev)
 {
-	if (tegra_pending_sgi()) {
+	if (tegra_pending_sgi() || need_resched()) {
 		/*
 		 * CPU got local interrupt that will be lost after GIC's
 		 * shutdown because GIC driver doesn't save/restore the
diff --git a/drivers/cpuidle/cpuidle.c b/drivers/cpuidle/cpuidle.c
index 4cc1ba49ce05..2bc52ccc339b 100644
--- a/drivers/cpuidle/cpuidle.c
+++ b/drivers/cpuidle/cpuidle.c
@@ -248,7 +248,7 @@ int cpuidle_enter_state(struct cpuidle_device *dev,
struct cpuidle_driver *drv,
 	 * Last need_resched() check must come after rcu_idle_enter()
 	 * which may wake up RCU internal tasks.
 	 */
-	if (!need_resched())
+	if ((target_state->flags & CPUIDLE_FLAG_COUPLED) || !need_resched())
 		entered_state = target_state->enter(dev, drv, index);
 	else
 		entered_state = -EBUSY;