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SubjectRe: [PATCH v2] sched/rt: Rework for_each_process_thread() iterations in tg_has_rt_tasks()
On 20/04/18 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.

This seems to be true. However, I wonder why we are OK with current racy
code (against tasks moving between groups). :/

Can't a task join the group while we are iterating and we miss that?

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