Re: [PATCH v2] sched/rt: Rework for_each_process_thread() iterations in tg_has_rt_tasks()

[Juri Lelli <juri.lelli@gmail.com>]

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?