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 17:30, Kirill Tkhai wrote:
> On 20.04.2018 17:11, Juri Lelli wrote:
> > 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?
> 
> Yes, it can, but I'm not sure either this should be considered as problem,
> seeing the race design we already have. It's not a real protection, this
> place is to warn a person, he does something wrong. We check for zero written
> there, but really written "1" will invent the same problems.

Mmm, right. :/