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Date:   Mon, 27 Jan 2020 11:30:18 -0500
From:   Phil Auld <pauld@redhat.com>
To:     Kirill Tkhai <ktkhai@virtuozzo.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@gmail.com>, mingo@redhat.com,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3]sched/rt: Stop for_each_process_thread() iterations in
 tg_has_rt_tasks()
Message-ID: <20200127163018.GB1295@pauld.bos.csb>
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On Fri, Jan 24, 2020 at 12:09:20PM +0300 Kirill Tkhai wrote:
> On 24.01.2020 00:56, Phil Auld wrote:
> > Hi,
> > 
> > On Thu, Apr 26, 2018 at 12:54:41PM +0300 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 |   21 +++++++++++----------
> >>  1 file changed, 11 insertions(+), 10 deletions(-)
> >>
> >> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> >> index 7aef6b4e885a..a40535c604b8 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 task_struct *task;
> >> +	struct css_task_iter it;
> >> +	int ret = 0;
> >>  
> >>  	/*
> >>  	 * Autogroups do not have RT tasks; see autogroup_create().
> >> @@ -2406,12 +2407,16 @@ 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 +2515,6 @@ 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 +2532,6 @@ 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 +2585,7 @@ 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;
> > 
> > Sorry to necro this...  
> > 
> > I have a customer case that has hit the issue here. It looks like this
> > fix didn't end up going in. 
> > 
> > The only way I could reproduce it myself was to add a udelay(2) to 
> > the tg_has_rt_tasks loop. With this patch applied, even with the 
> > udelay nothing bad happens.
> > 
> > Kirill, did you have a good way to reproduce it without adding 
> > delays? 
> 
> I have no a reproducer, it just used to fire on some of our customer nodes.
> It depends on many parameters, and main is workload. Also, I think, interrupts
> affinity may be involved (whether they bound to small subset of cpus, or they
> distributed over all cores). If this is possible theoretically, it occurs
> practically with some probability.
> 
> We fixed this with out-of-tree patch in our vzkernel tree, so we haven't seen
> a reproduction anymore.
> 

Thanks Kirill. 


Fwiw, it looks like Konstantin Khlebnikov has picked up this change in 
a newer patch, so if that can get in then this would be cleared up too.

https://lkml.org/lkml/2020/1/25/167


Cheers,
Phil


> > Peter, is there any chance of taking something like this?
> > 
> > Is there a better fix? 
> 

--