[Devel] [PATCH rh7] sched: Fix race on toggling cfs_bandwidth_used

[Kirill Tkhai]

On 30.08.2016 17:58, Andrey Ryabinin wrote:
> From: Ben Segall <bsegall at google.com>
> 
> commit 1ee14e6c8cddeeb8a490d7b54cd9016e4bb900b4 upstream.
> 
> When we transition cfs_bandwidth_used to false, any currently
> throttled groups will incorrectly return false from cfs_rq_throttled.
> While tg_set_cfs_bandwidth will unthrottle them eventually, currently
> running code (including at least dequeue_task_fair and
> distribute_cfs_runtime) will cause errors.
> 
> Fix this by turning off cfs_bandwidth_used only after unthrottling all
> cfs_rqs.
> 
> Tested: toggle bandwidth back and forth on a loaded cgroup. Caused
> crashes in minutes without the patch, hasn't crashed with it.
> 
> Signed-off-by: Ben Segall <bsegall at google.com>
> Signed-off-by: Peter Zijlstra <peterz at infradead.org>
> Cc: pjt at google.com
> Link: http://lkml.kernel.org/r/20131016181611.22647.80365.stgit@sword-of-the-dawn.mtv.corp.google.com
> Signed-off-by: Ingo Molnar <mingo at kernel.org>
> 
> https://jira.sw.ru/browse/PSBM-51056
> 
> Signed-off-by: Andrey Ryabinin <aryabinin at virtuozzo.com>

Acked-by: Kirill Tkhai <ktkhai at virtuozzo.com>

> ---
>  kernel/sched/core.c  |  9 ++++++++-
>  kernel/sched/fair.c  | 16 +++++++++-------
>  kernel/sched/sched.h |  3 ++-
>  3 files changed, 19 insertions(+), 9 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 657b8e4..ccc826a 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -8470,7 +8470,12 @@ static int __tg_set_cfs_bandwidth(struct task_group *tg, u64 period, u64 quota)
>  
>  	runtime_enabled = quota != RUNTIME_INF;
>  	runtime_was_enabled = cfs_b->quota != RUNTIME_INF;
> -	account_cfs_bandwidth_used(runtime_enabled, runtime_was_enabled);
> +	/*
> +	 * If we need to toggle cfs_bandwidth_used, off->on must occur
> +	 * before making related changes, and on->off must occur afterwards
> +	 */
> +	if (runtime_enabled && !runtime_was_enabled)
> +		cfs_bandwidth_usage_inc();
>  	raw_spin_lock_irq(&cfs_b->lock);
>  	cfs_b->period = ns_to_ktime(period);
>  	cfs_b->quota = quota;
> @@ -8498,6 +8503,8 @@ static int __tg_set_cfs_bandwidth(struct task_group *tg, u64 period, u64 quota)
>  	}
>  	if (runtime_enabled != runtime_was_enabled)
>  		tg_limit_toggled(tg);
> +	if (runtime_was_enabled && !runtime_enabled)
> +		cfs_bandwidth_usage_dec();
>  	return ret;
>  }
>  
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index a419f39..b4da77d 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -422,13 +422,14 @@ static inline bool cfs_bandwidth_used(void)
>  	return static_key_false(&__cfs_bandwidth_used);
>  }
>  
> -void account_cfs_bandwidth_used(int enabled, int was_enabled)
> +void cfs_bandwidth_usage_inc(void)
>  {
> -	/* only need to count groups transitioning between enabled/!enabled */
> -	if (enabled && !was_enabled)
> -		static_key_slow_inc(&__cfs_bandwidth_used);
> -	else if (!enabled && was_enabled)
> -		static_key_slow_dec(&__cfs_bandwidth_used);
> +	static_key_slow_inc(&__cfs_bandwidth_used);
> +}
> +
> +void cfs_bandwidth_usage_dec(void)
> +{
> +	static_key_slow_dec(&__cfs_bandwidth_used);
>  }
>  #else /* HAVE_JUMP_LABEL */
>  static bool cfs_bandwidth_used(void)
> @@ -436,7 +437,8 @@ static bool cfs_bandwidth_used(void)
>  	return true;
>  }
>  
> -void account_cfs_bandwidth_used(int enabled, int was_enabled) {}
> +void cfs_bandwidth_usage_inc(void) {}
> +void cfs_bandwidth_usage_dec(void) {}
>  #endif /* HAVE_JUMP_LABEL */
>  
>  static inline int cfs_rq_throttled(struct cfs_rq *cfs_rq)
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index d9fe825..894260d 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1444,7 +1444,8 @@ extern void print_rt_stats(struct seq_file *m, int cpu);
>  extern void init_cfs_rq(struct cfs_rq *cfs_rq);
>  extern void init_rt_rq(struct rt_rq *rt_rq, struct rq *rq);
>  
> -extern void account_cfs_bandwidth_used(int enabled, int was_enabled);
> +extern void cfs_bandwidth_usage_inc(void);
> +extern void cfs_bandwidth_usage_dec(void);
>  
>  #ifdef CONFIG_FAIR_GROUP_SCHED
>  static inline struct cfs_rq *task_cfs_rq(struct task_struct *p)
>