Re: [PATCH v2] sched: Fix race between runtime distribution and assignment

[bsegall]

Huaixin Chang <changhuaixin@xxxxxxxxxxxxxxxxx> writes:

> Currently, there is a potential race between distribute_cfs_runtime()
> and assign_cfs_rq_runtime(). Race happens when cfs_b->runtime is read,
> distributes without holding lock and finds out there is not enough
> runtime to charge against after distribution. Because
> assign_cfs_rq_runtime() might be called during distribution, and use
> cfs_b->runtime at the same time.
>
> Fibtest is the tool to test this race. Assume all gcfs_rq is throttled
> and cfs period timer runs, slow threads might run and sleep, returning
> unused cfs_rq runtime and keeping min_cfs_rq_runtime in their local
> pool. If all this happens sufficiently quickly, cfs_b->runtime will drop
> a lot. If runtime distributed is large too, over-use of runtime happens.
>
> A runtime over-using by about 70 percent of quota is seen when we
> test fibtest on a 96-core machine. We run fibtest with 1 fast thread and
> 95 slow threads in test group, configure 10ms quota for this group and
> see the CPU usage of fibtest is 17.0%, which is far more than the
> expected 10%.
>
> On a smaller machine with 32 cores, we also run fibtest with 96
> threads. CPU usage is more than 12%, which is also more than expected
> 10%. This shows that on similar workloads, this race do affect CPU
> bandwidth control.
>
> Solve this by holding lock inside distribute_cfs_runtime().

Some minor requests below, other than that

Reviewed-by: Ben Segall <bsegall@xxxxxxxxxx>

>
> Fixes: c06f04c70489 ("sched: Fix potential near-infinite distribute_cfs_runtime() loop")
> Signed-off-by: Huaixin Chang <changhuaixin@xxxxxxxxxxxxxxxxx>
> ---
> v2: fix spelling, initialize variable rumaining in distribute_cfs_runtime()
> ---
>  kernel/sched/fair.c | 31 ++++++++++++-------------------
>  1 file changed, 12 insertions(+), 19 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index c1217bfe5e81..fd30e06a7ffc 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4629,11 +4629,11 @@ void unthrottle_cfs_rq(struct cfs_rq *cfs_rq)
>  		resched_curr(rq);
>  }
>  
> -static u64 distribute_cfs_runtime(struct cfs_bandwidth *cfs_b, u64 remaining)
> +static void distribute_cfs_runtime(struct cfs_bandwidth *cfs_b)
>  {
>  	struct cfs_rq *cfs_rq;
> -	u64 runtime;
> -	u64 starting_runtime = remaining;
> +	u64 runtime, remaining = 1;
> +	unsigned long flags;
>  
>  	rcu_read_lock();
>  	list_for_each_entry_rcu(cfs_rq, &cfs_b->throttled_cfs_rq,
> @@ -4648,10 +4648,13 @@ static u64 distribute_cfs_runtime(struct cfs_bandwidth *cfs_b, u64 remaining)
>  		/* By the above check, this should never be true */
>  		SCHED_WARN_ON(cfs_rq->runtime_remaining > 0);
>  
> +		raw_spin_lock_irqsave(&cfs_b->lock, flags);

No need for _irqsave/_irqrestore, the rqlock already did.

>  		runtime = -cfs_rq->runtime_remaining + 1;
> -		if (runtime > remaining)
> -			runtime = remaining;
> -		remaining -= runtime;
> +		if (runtime > cfs_b->runtime)
> +			runtime = cfs_b->runtime;
> +		cfs_b->runtime -= runtime;
> +		remaining = cfs_b->runtime;
> +		raw_spin_unlock_irqrestore(&cfs_b->lock, flags);
>  
>  		cfs_rq->runtime_remaining += runtime;
>  
> @@ -4666,8 +4669,6 @@ static u64 distribute_cfs_runtime(struct cfs_bandwidth *cfs_b, u64 remaining)
>  			break;
>  	}
>  	rcu_read_unlock();
> -
> -	return starting_runtime - remaining;
>  }
>  
>  /*
> @@ -4678,7 +4679,6 @@ static u64 distribute_cfs_runtime(struct cfs_bandwidth *cfs_b, u64 remaining)
>   */
>  static int do_sched_cfs_period_timer(struct cfs_bandwidth *cfs_b, int overrun, unsigned long flags)
>  {
> -	u64 runtime;
>  	int throttled;
>  
>  	/* no need to continue the timer with no bandwidth constraint */
> @@ -4708,23 +4708,17 @@ static int do_sched_cfs_period_timer(struct cfs_bandwidth *cfs_b, int overrun, u
>  
>  	/*
>  	 * This check is repeated as we are holding onto the new bandwidth while
> -	 * we unthrottle. This can potentially race with an unthrottled group
> -	 * trying to acquire new bandwidth from the global pool. This can result
> -	 * in us over-using our runtime if it is all used during this loop, but
> -	 * only by limited amounts in that extreme case.
> +	 * we unthrottle.

"This check is repeated as we release cfs_b->lock while we unthrottle."
or something like that. This code is no longer even holding onto the new
bandwidth on its own.

>  	 */
>  	while (throttled && cfs_b->runtime > 0 && !cfs_b->distribute_running) {
> -		runtime = cfs_b->runtime;
>  		cfs_b->distribute_running = 1;
>  		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);
> +		distribute_cfs_runtime(cfs_b);
>  		raw_spin_lock_irqsave(&cfs_b->lock, flags);
>  
>  		cfs_b->distribute_running = 0;
>  		throttled = !list_empty(&cfs_b->throttled_cfs_rq);
> -
> -		lsub_positive(&cfs_b->runtime, runtime);
>  	}
>  
>  	/*
> @@ -4858,10 +4852,9 @@ static void do_sched_cfs_slack_timer(struct cfs_bandwidth *cfs_b)
>  	if (!runtime)
>  		return;
>  
> -	runtime = distribute_cfs_runtime(cfs_b, runtime);
> +	distribute_cfs_runtime(cfs_b);
>  
>  	raw_spin_lock_irqsave(&cfs_b->lock, flags);
> -	lsub_positive(&cfs_b->runtime, runtime);
>  	cfs_b->distribute_running = 0;
>  	raw_spin_unlock_irqrestore(&cfs_b->lock, flags);
>  }