lkml.org 
[lkml]   [2018]   [Apr]   [20]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH] sched/rt: Rework for_each_process_thread() iterations in tg_has_rt_tasks()
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)?

Best,

- Juri

\
 
 \ /
  Last update: 2018-04-20 11:26    [W:0.230 / U:0.720 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site