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Date:   Thu, 30 Jan 2020 14:10:13 -0500
From:   Phil Auld <pauld@redhat.com>
To:     linux-kernel@vger.kernel.org
Cc:     linux-tip-commits@vger.kernel.org,
        Konstantin Khlebnikov <khlebnikov@yandex-team.ru>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>, x86 <x86@kernel.org>
Subject: Re: [tip: sched/core] sched/rt: Optimize checking group RT scheduler
 constraints
Message-ID: <20200130191013.GA24632@pauld.bos.csb>
References: <157996383820.4651.11292439232549211693.stgit@buzz>
 <158029757654.396.14560704856092377008.tip-bot2@tip-bot2>
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On Wed, Jan 29, 2020 at 11:32:56AM -0000 tip-bot2 for Konstantin Khlebnikov wrote:
> The following commit has been merged into the sched/core branch of tip:
> 
> Commit-ID:     b4fb015eeff7f3e5518a7dbe8061169a3e2f2bc7
> Gitweb:        https://git.kernel.org/tip/b4fb015eeff7f3e5518a7dbe8061169a3e2f2bc7
> Author:        Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
> AuthorDate:    Sat, 25 Jan 2020 17:50:38 +03:00
> Committer:     Ingo Molnar <mingo@kernel.org>
> CommitterDate: Tue, 28 Jan 2020 21:37:09 +01:00
> 
> sched/rt: Optimize checking group RT scheduler constraints
> 
> 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>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Signed-off-by: Ingo Molnar <mingo@kernel.org>
> Link: https://lkml.kernel.org/r/157996383820.4651.11292439232549211693.stgit@buzz
> ---
>  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 4043abe..55a4a50 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -2449,10 +2449,11 @@ 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().
> @@ -2460,12 +2461,12 @@ 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);
>  

Heh, I misread it the first time and didn't see the !ret condition. But I think 
it should be just "ret = ".

But thanks for getting this fixed.


Cheers,
Phil


> -	return 0;
> +	return ret;
>  }
>  
>  struct rt_schedulable_data {
> @@ -2496,9 +2497,10 @@ 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);
> @@ -2564,7 +2566,6 @@ 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;
> @@ -2582,7 +2583,6 @@ 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;
> @@ -2641,9 +2641,7 @@ 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;
> 

--