On 20/04/18 13:06, Kirill Tkhai wrote: > From: Kirill Tkhai <[email protected]> > > 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?
