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

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@kernel.org Fixes: 1098582a0f6c (sched,idle,rcu: Push rcu_idle deeper into the idle path) Cc: stable@vger.kernel.org Cc: Daniel Lezcano daniel.lezcano@linaro.org Cc: Peter Zijlstra peterz@infradead.org Cc: Rafael J. Wysocki rafael.j.wysocki@intel.com Cc: Thomas Gleixner tglx@linutronix.de Cc: Ingo Molnar mingo@kernel.org Signed-off-by: Frederic Weisbecker frederic@kernel.org

---

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.