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Date:   Mon, 27 Jan 2020 12:47:49 -0500
From:   Phil Auld <pauld@redhat.com>
To:     Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Cc:     linux-kernel@vger.kernel.org, Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>,
        Ingo Molnar <mingo@redhat.com>, Mel Gorman <mgorman@suse.de>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>
Subject: Re: [PATCH] sched/rt: optimize checking group rt scheduler
 constraints
Message-ID: <20200127174748.GE1295@pauld.bos.csb>
References: <157996383820.4651.11292439232549211693.stgit@buzz>
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Hi Konstantin,

On Sat, Jan 25, 2020 at 05:50:38PM +0300 Konstantin Khlebnikov wrote:
> Group RT scheduler contains protection against setting zero runtime for
> cgroup with rt tasks. Right now function tg_set_rt_bandwidth() iterates
> over all cpu cgroups and calls tg_has_rt_tasks() for any cgroup which
> runtime is zero (not only for changed one). Default rt runtime is zero,
> thus tg_has_rt_tasks() will is called for almost at cpu cgroups.
> 
> This protection already is slightly racy: runtime limit could be changed
> between cpu_cgroup_can_attach() and cpu_cgroup_attach() because changing
> cgroup attribute does not lock cgroup_mutex while attach does not lock
> rt_constraints_mutex. Changing task scheduler class also races with
> changing rt runtime: check in __sched_setscheduler() isn't protected.
> 
> Function tg_has_rt_tasks() iterates over all threads in the system.
> This gives NR_CGROUPS * NR_TASKS operations under single tasklist_lock
> locked for read tg_set_rt_bandwidth(). Any concurrent attempt of locking
> tasklist_lock for write (for example fork) will stuck with disabled irqs.
> 
> This patch makes two optimizations:
> 1) Remove locking tasklist_lock and iterate only tasks in cgroup
> 2) Call tg_has_rt_tasks() iff rt runtime changes from non-zero to zero
> 
> All changed code is under CONFIG_RT_GROUP_SCHED.
> 
> Testcase:
> # mkdir /sys/fs/cgroup/cpu/test{1..10000}
> # echo 0 | tee /sys/fs/cgroup/cpu/test*/cpu.rt_runtime_us
> 
> At the same time without patch fork time will be >100ms:
> # perf trace -e clone --duration 100 stress-ng --fork 1
> 
> Also remote ping will show timings >100ms caused by irq latency.
> 
> Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
> ---
>  kernel/sched/rt.c |   24 +++++++++++-------------
>  1 file changed, 11 insertions(+), 13 deletions(-)
> 
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index e591d40fd645..95d1d7be84ef 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -2396,10 +2396,11 @@ const struct sched_class rt_sched_class = {
>   */
>  static DEFINE_MUTEX(rt_constraints_mutex);
>  
> -/* Must be called with tasklist_lock held */
>  static inline int tg_has_rt_tasks(struct task_group *tg)
>  {
> -	struct task_struct *g, *p;
> +	struct task_struct *task;
> +	struct css_task_iter it;
> +	int ret = 0;
>  
>  	/*
>  	 * Autogroups do not have RT tasks; see autogroup_create().
> @@ -2407,12 +2408,12 @@ static inline int tg_has_rt_tasks(struct task_group *tg)
>  	if (task_group_is_autogroup(tg))
>  		return 0;
>  
> -	for_each_process_thread(g, p) {
> -		if (rt_task(p) && task_group(p) == tg)
> -			return 1;
> -	}
> +	css_task_iter_start(&tg->css, 0, &it);
> +	while (!ret && (task = css_task_iter_next(&it)))
> +		ret |= rt_task(task);
> +	css_task_iter_end(&it);
>  

Why not a break; in there alon with the change to "="?


> -	return 0;
> +	return ret;
>  }
>  
>  struct rt_schedulable_data {
> @@ -2443,9 +2444,10 @@ static int tg_rt_schedulable(struct task_group *tg, void *data)
>  		return -EINVAL;
>  
>  	/*
> -	 * Ensure we don't starve existing RT tasks.
> +	 * Ensure we don't starve existing RT tasks if runtime turns zero.
>  	 */
> -	if (rt_bandwidth_enabled() && !runtime && tg_has_rt_tasks(tg))
> +	if (rt_bandwidth_enabled() && !runtime &&
> +	    tg->rt_bandwidth.rt_runtime && tg_has_rt_tasks(tg))
>  		return -EBUSY;
>  
>  	total = to_ratio(period, runtime);
> @@ -2511,7 +2513,6 @@ static int tg_set_rt_bandwidth(struct task_group *tg,
>  		return -EINVAL;
>  
>  	mutex_lock(&rt_constraints_mutex);
> -	read_lock(&tasklist_lock);
>  	err = __rt_schedulable(tg, rt_period, rt_runtime);
>  	if (err)
>  		goto unlock;
> @@ -2529,7 +2530,6 @@ static int tg_set_rt_bandwidth(struct task_group *tg,
>  	}
>  	raw_spin_unlock_irq(&tg->rt_bandwidth.rt_runtime_lock);
>  unlock:
> -	read_unlock(&tasklist_lock);
>  	mutex_unlock(&rt_constraints_mutex);
>  
>  	return err;
> @@ -2588,9 +2588,7 @@ static int sched_rt_global_constraints(void)
>  	int ret = 0;
>  
>  	mutex_lock(&rt_constraints_mutex);
> -	read_lock(&tasklist_lock);
>  	ret = __rt_schedulable(NULL, 0, 0);
> -	read_unlock(&tasklist_lock);
>  	mutex_unlock(&rt_constraints_mutex);
>  
>  	return ret;
> 


Thanks,
Phil

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