Messages in this thread | | | Date | Mon, 27 Jan 2020 11:30:18 -0500 | From | Phil Auld <> | Subject | Re: [PATCH v3]sched/rt: Stop for_each_process_thread() iterations in tg_has_rt_tasks() |
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On Fri, Jan 24, 2020 at 12:09:20PM +0300 Kirill Tkhai wrote: > On 24.01.2020 00:56, Phil Auld wrote: > > Hi, > > > > On Thu, Apr 26, 2018 at 12:54:41PM +0300 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. > >> > >> Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com> > >> --- > >> kernel/sched/rt.c | 21 +++++++++++---------- > >> 1 file changed, 11 insertions(+), 10 deletions(-) > >> > >> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c > >> index 7aef6b4e885a..a40535c604b8 100644 > >> --- a/kernel/sched/rt.c > >> +++ b/kernel/sched/rt.c > >> @@ -2395,10 +2395,11 @@ 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 task_struct *task; > >> + struct css_task_iter it; > >> + int ret = 0; > >> > >> /* > >> * Autogroups do not have RT tasks; see autogroup_create(). > >> @@ -2406,12 +2407,16 @@ 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; > >> + css_task_iter_start(&tg->css, 0, &it); > >> + while ((task = css_task_iter_next(&it))) { > >> + if (rt_task(task)) { > >> + ret = 1; > >> + break; > >> + } > >> } > >> + css_task_iter_end(&it); > >> > >> - return 0; > >> + return ret; > >> } > >> > >> struct rt_schedulable_data { > >> @@ -2510,7 +2515,6 @@ static int tg_set_rt_bandwidth(struct task_group *tg, > >> return -EINVAL; > >> > >> mutex_lock(&rt_constraints_mutex); > >> - read_lock(&tasklist_lock); > >> err = __rt_schedulable(tg, rt_period, rt_runtime); > >> if (err) > >> goto unlock; > >> @@ -2528,7 +2532,6 @@ static int tg_set_rt_bandwidth(struct task_group *tg, > >> } > >> raw_spin_unlock_irq(&tg->rt_bandwidth.rt_runtime_lock); > >> unlock: > >> - read_unlock(&tasklist_lock); > >> mutex_unlock(&rt_constraints_mutex); > >> > >> return err; > >> @@ -2582,9 +2585,7 @@ static int sched_rt_global_constraints(void) > >> int ret = 0; > >> > >> mutex_lock(&rt_constraints_mutex); > >> - read_lock(&tasklist_lock); > >> ret = __rt_schedulable(NULL, 0, 0); > >> - read_unlock(&tasklist_lock); > >> mutex_unlock(&rt_constraints_mutex); > >> > >> return ret; > > > > Sorry to necro this... > > > > I have a customer case that has hit the issue here. It looks like this > > fix didn't end up going in. > > > > The only way I could reproduce it myself was to add a udelay(2) to > > the tg_has_rt_tasks loop. With this patch applied, even with the > > udelay nothing bad happens. > > > > Kirill, did you have a good way to reproduce it without adding > > delays? > > I have no a reproducer, it just used to fire on some of our customer nodes. > It depends on many parameters, and main is workload. Also, I think, interrupts > affinity may be involved (whether they bound to small subset of cpus, or they > distributed over all cores). If this is possible theoretically, it occurs > practically with some probability. > > We fixed this with out-of-tree patch in our vzkernel tree, so we haven't seen > a reproduction anymore. >
Thanks Kirill.
Fwiw, it looks like Konstantin Khlebnikov has picked up this change in a newer patch, so if that can get in then this would be cleared up too.
https://lkml.org/lkml/2020/1/25/167
Cheers, Phil
> > Peter, is there any chance of taking something like this? > > > > Is there a better fix? >
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