Re: [PATCH] sched/rt: Rework for_each_process_thread() iterations in tg_has_rt_tasks()

[Kirill Tkhai <ktkhai@virtuozzo.com>]

Hi, Juri,

On 20.04.2018 12:25, Juri Lelli wrote:
> Hi Kirill,
> 
> On 19/04/18 20:29, Kirill Tkhai wrote:
>> 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 check
>> for rt_rq::rt_nr_running instead of iteration over task list.
>> This makes the function to scale well, and its execution time
>> does not depend on number of processes in the system.
>>
>> 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. Also, rt_rq::rt_nr_running contains queued
>> child cfs_rq in additional to queued task. Since tg_has_rt_tasks()
> 
> s/cfs_/rt_/ , right?
> 
>> is used in case of !runtime case:
>>
>> 	if (rt_bandwidth_enabled() && !runtime && tg_has_rt_tasks(tg))
>> 		return -EBUSY;
>>
>> the behaviour won't change. The only change is that walk_tg_tree()
>> calling tg_rt_schedulable() will break its iteration on parent cfs_rq,
> 
> Ditto.
> 
>> i.e. earlier.
>>
>> Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
>> ---
>>  kernel/sched/rt.c |   22 ++++++++++++----------
>>  1 file changed, 12 insertions(+), 10 deletions(-)
>>
>> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
>> index 7aef6b4e885a..601151bb9322 100644
>> --- a/kernel/sched/rt.c
>> +++ b/kernel/sched/rt.c
>> @@ -2395,10 +2395,10 @@ 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 rt_rq *rt_rq;
>> +	int cpu, ret = 0;
>>  
>>  	/*
>>  	 * Autogroups do not have RT tasks; see autogroup_create().
>> @@ -2406,12 +2406,18 @@ 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;
>> +	preempt_disable();
>> +
>> +	for_each_online_cpu(cpu) {
>> +		rt_rq = tg->rt_rq[cpu];
>> +		if (READ_ONCE(rt_rq->rt_nr_running)) {
> 
> Isn't this however checking against the current (dynamic) number of
> runnable tasks/groups instead of the "static" group membership (which
> shouldn't be affected by a task running state)?

Ah, you are sure. I forgot that rt_nr_running does not contain sleeping tasks.
We need to check something else here. I'll try to find another way.

Thanks,
Kirill