Re: CFS scheduler: spin_lock usage causes dead lock when smp_apic_timer_interrupt occurs

[Mike Galbraith <efault@gmx.de>]

On Mon, 2019-01-07 at 13:52 +0100, Peter Zijlstra wrote:
> On Mon, Jan 07, 2019 at 01:28:34PM +0100, Mike Galbraith wrote:
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 960ad0ce77d7..420624c49f38 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -5007,9 +5007,9 @@ void init_cfs_bandwidth(struct cfs_bandwidth *cfs_b)
> >  	cfs_b->period = ns_to_ktime(default_cfs_period());
> >  
> >  	INIT_LIST_HEAD(&cfs_b->throttled_cfs_rq);
> > -	hrtimer_init(&cfs_b->period_timer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS_PINNED);
> > +	hrtimer_init(&cfs_b->period_timer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS_PINNED_HARD);
> >  	cfs_b->period_timer.function = sched_cfs_period_timer;
> > -	hrtimer_init(&cfs_b->slack_timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
> > +	hrtimer_init(&cfs_b->slack_timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL_HARD);
> >  	cfs_b->slack_timer.function = sched_cfs_slack_timer;
> >  }
> 
> Right, that should sort it. But I'm not sure this is the best solution
> though. That cfs-runtime crud can (IIRC) iterate lists etc.. so running
> it from the softirq isn't a bad idea. We just need to fix that locking
> up a bit.
> 
> Something a wee bit like so perhaps..

I plugged that into 4.19-rt along with revert of hard irq context, and
it (as expected) does the trick.

> ---
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 13776fac7b74..3cfe26aa098a 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4566,7 +4566,7 @@ static u64 distribute_cfs_runtime(struct cfs_bandwidth *cfs_b,
>  		struct rq *rq = rq_of(cfs_rq);
>  		struct rq_flags rf;
>  
> -		rq_lock(rq, &rf);
> +		rq_lock_irqsave(rq, &rf);
>  		if (!cfs_rq_throttled(cfs_rq))
>  			goto next;
>  
> @@ -4583,7 +4583,7 @@ static u64 distribute_cfs_runtime(struct cfs_bandwidth *cfs_b,
>  			unthrottle_cfs_rq(cfs_rq);
>  
>  next:
> -		rq_unlock(rq, &rf);
> +		rq_unlock_irqrestore(rq, &rf);
>  
>  		if (!remaining)
>  			break;
> @@ -4599,7 +4599,7 @@ static u64 distribute_cfs_runtime(struct cfs_bandwidth *cfs_b,
>   * period the timer is deactivated until scheduling resumes; cfs_b->idle is
>   * used to track this state.
>   */
> -static int do_sched_cfs_period_timer(struct cfs_bandwidth *cfs_b, int overrun)
> +static int do_sched_cfs_period_timer(struct cfs_bandwidth *cfs_b, int overrun, unsigned long flags)
>  {
>  	u64 runtime, runtime_expires;
>  	int throttled;
> @@ -4641,11 +4641,11 @@ static int do_sched_cfs_period_timer(struct cfs_bandwidth *cfs_b, int overrun)
>  	while (throttled && cfs_b->runtime > 0 && !cfs_b->distribute_running) {
>  		runtime = cfs_b->runtime;
>  		cfs_b->distribute_running = 1;
> -		raw_spin_unlock(&cfs_b->lock);
> +		raw_spin_unlock_irqrestore(&cfs_b->lock, flags);
>  		/* we can't nest cfs_b->lock while distributing bandwidth */
>  		runtime = distribute_cfs_runtime(cfs_b, runtime,
>  						 runtime_expires);
> -		raw_spin_lock(&cfs_b->lock);
> +		raw_spin_lock_irqsave(&cfs_b->lock, flags);
>  
>  		cfs_b->distribute_running = 0;
>  		throttled = !list_empty(&cfs_b->throttled_cfs_rq);
> @@ -4754,17 +4754,18 @@ static __always_inline void return_cfs_rq_runtime(struct cfs_rq *cfs_rq)
>  static void do_sched_cfs_slack_timer(struct cfs_bandwidth *cfs_b)
>  {
>  	u64 runtime = 0, slice = sched_cfs_bandwidth_slice();
> +	unsigned long flags;
>  	u64 expires;
>  
>  	/* confirm we're still not at a refresh boundary */
> -	raw_spin_lock(&cfs_b->lock);
> +	raw_spin_lock_irqsave(&cfs_b->lock, flags);
>  	if (cfs_b->distribute_running) {
> -		raw_spin_unlock(&cfs_b->lock);
> +		raw_spin_unlock_irqrestore(&cfs_b->lock, flags);
>  		return;
>  	}
>  
>  	if (runtime_refresh_within(cfs_b, min_bandwidth_expiration)) {
> -		raw_spin_unlock(&cfs_b->lock);
> +		raw_spin_unlock_irqrestore(&cfs_b->lock, flags);
>  		return;
>  	}
>  
> @@ -4775,18 +4776,18 @@ static void do_sched_cfs_slack_timer(struct cfs_bandwidth *cfs_b)
>  	if (runtime)
>  		cfs_b->distribute_running = 1;
>  
> -	raw_spin_unlock(&cfs_b->lock);
> +	raw_spin_unlock_irqrestore(&cfs_b->lock, flags);
>  
>  	if (!runtime)
>  		return;
>  
>  	runtime = distribute_cfs_runtime(cfs_b, runtime, expires);
>  
> -	raw_spin_lock(&cfs_b->lock);
> +	raw_spin_lock_irqsave(&cfs_b->lock, flags);
>  	if (expires == cfs_b->runtime_expires)
>  		lsub_positive(&cfs_b->runtime, runtime);
>  	cfs_b->distribute_running = 0;
> -	raw_spin_unlock(&cfs_b->lock);
> +	raw_spin_unlock_irqrestore(&cfs_b->lock, flags);
>  }
>  
>  /*
> @@ -4864,20 +4865,21 @@ static enum hrtimer_restart sched_cfs_period_timer(struct hrtimer *timer)
>  {
>  	struct cfs_bandwidth *cfs_b =
>  		container_of(timer, struct cfs_bandwidth, period_timer);
> +	unsigned long flags;
>  	int overrun;
>  	int idle = 0;
>  
> -	raw_spin_lock(&cfs_b->lock);
> +	raw_spin_lock_irqsave(&cfs_b->lock, flags);
>  	for (;;) {
>  		overrun = hrtimer_forward_now(timer, cfs_b->period);
>  		if (!overrun)
>  			break;
>  
> -		idle = do_sched_cfs_period_timer(cfs_b, overrun);
> +		idle = do_sched_cfs_period_timer(cfs_b, overrun, flags);
>  	}
>  	if (idle)
>  		cfs_b->period_active = 0;
> -	raw_spin_unlock(&cfs_b->lock);
> +	raw_spin_unlock_irqrestore(&cfs_b->lock, flags);
>  
>  	return idle ? HRTIMER_NORESTART : HRTIMER_RESTART;
>  }