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Subject: Re: [PATCH v2] sched/rt: Rework for_each_process_thread() iterations
 in tg_has_rt_tasks()
From:   Kirill Tkhai <ktkhai@virtuozzo.com>
To:     Juri Lelli <juri.lelli@gmail.com>, mingo@redhat.com,
        peterz@infradead.org
Cc:     linux-kernel@vger.kernel.org
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Date:   Wed, 25 Apr 2018 18:42:11 +0300
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Peter,

could you please to give some comments on this?

Kirill

On 20.04.2018 13:06, Kirill Tkhai wrote:
> From: Kirill Tkhai <ktkhai@virtuozzo.com>
> 
> tg_rt_schedulable() iterates over all child task groups,
> while tg_has_rt_tasks() iterates over all linked tasks.
> In case of systems with big number of tasks, this may
> take a lot of time.
> 
> I observed hard LOCKUP on machine with 20000+ processes
> after write to "cpu.rt_period_us" of cpu cgroup with
> 39 children. The problem occurred because of tasklist_lock
> is held for a long time and other processes can't do fork().
> 
> PID: 1036268  TASK: ffff88766c310000  CPU: 36  COMMAND: "criu"
>  #0 [ffff887f7f408e48] crash_nmi_callback at ffffffff81050601
>  #1 [ffff887f7f408e58] nmi_handle at ffffffff816e0cc7
>  #2 [ffff887f7f408eb0] do_nmi at ffffffff816e0fb0
>  #3 [ffff887f7f408ef0] end_repeat_nmi at ffffffff816e00b9
>     [exception RIP: tg_rt_schedulable+463]
>     RIP: ffffffff810bf49f  RSP: ffff886537ad7d50  RFLAGS: 00000202
>     RAX: 0000000000000000  RBX: 000000003b9aca00  RCX: ffff883e9cb4b1b0
>     RDX: ffff887d0be43608  RSI: ffff886537ad7dd8  RDI: ffff8840a6ad0000
>     RBP: ffff886537ad7d68   R8: ffff887d0be431b0   R9: 00000000000e7ef0
>     R10: ffff88164fc39400  R11: 0000000000023380  R12: ffffffff81ef8d00
>     R13: ffffffff810bea40  R14: 0000000000000000  R15: ffff8840a6ad0000
>     ORIG_RAX: ffffffffffffffff  CS: 0010  SS: 0018
> --- <NMI exception stack> ---
>  #4 [ffff886537ad7d50] tg_rt_schedulable at ffffffff810bf49f
>  #5 [ffff886537ad7d70] walk_tg_tree_from at ffffffff810c6c91
>  #6 [ffff886537ad7dc0] tg_set_rt_bandwidth at ffffffff810c6dd0
>  #7 [ffff886537ad7e28] cpu_rt_period_write_uint at ffffffff810c6eea
>  #8 [ffff886537ad7e38] cgroup_file_write at ffffffff8111cfd3
>  #9 [ffff886537ad7ec8] vfs_write at ffffffff8121eced
> #10 [ffff886537ad7f08] sys_write at ffffffff8121faff
> #11 [ffff886537ad7f50] system_call_fastpath at ffffffff816e8a7d
> 
> The patch reworks tg_has_rt_tasks() and makes it to iterate over
> task group process list instead of iteration over all tasks list.
> This makes the function to scale well, and reduces its execution
> time.
> 
> Note, that since tasklist_lock doesn't protect a task against
> sched_class changing, we don't introduce new races in comparison
> to that we had before.
> 
> Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
> ---
>  kernel/sched/rt.c |   22 +++++++++++-----------
>  1 file changed, 11 insertions(+), 11 deletions(-)
> 
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index 7aef6b4e885a..20be796d4e63 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -2395,10 +2395,11 @@ 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 css_task_iter it;
> +	struct task_struct *task;
> +	int ret = 0;
>  
>  	/*
>  	 * Autogroups do not have RT tasks; see autogroup_create().
> @@ -2406,12 +2407,15 @@ 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 ((task = css_task_iter_next(&it)))
> +		if (rt_task(task)) {
> +			ret = 1;
> +			break;
> +		}
> +	css_task_iter_end(&it);
>  
> -	return 0;
> +	return ret;
>  }
>  
>  struct rt_schedulable_data {
> @@ -2510,7 +2514,6 @@ 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;
> @@ -2528,7 +2531,6 @@ 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;
> @@ -2582,9 +2584,7 @@ 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;
>