Hi Valentin,
On Mon, Mar 18, 2024 at 6:34 PM Valentin Schneider <vschneid@xxxxxxxxxx> wrote:
> On 18/03/24 12:17, Daniel Vacek wrote:
> > Bill Peters reported CPU hangs while offlining/onlining CPUs on s390.
> >
> > Analyzing the vmcore data shows `stop_one_cpu_nowait()` in `affine_move_task()`
> > can fail when racing with off-/on-lining resulting in a deadlock waiting for
> > the pending migration stop work completion which is never done.
> >
> > Fix this by correctly handling such a condition.
> >
>
> IIUC the problem is that the dest_cpu and its stopper thread can be taken
> down by take_cpu_down(), and affine_move_task() currently isn't aware of
> that. I thought we had tested this vs hotplug, but oh well...
I'm sorry, I should have provided more context in the first place. The machine
is an LPAR with 2 CPUs and CPU 0 was onlining (hotplugging?) CPU 1. The traces
show this scenario:
CPU 0 | CPU 1
|
cpuplugd task 1429 |
holds the `cpu_hotplug_lock` |
for writing in _cpu_up+0x16a |
blocked on `cpuhp_state:1.done_up` |
completion in __cpuhp_kick_ap+0x76 |
|
| cpuhp/1 task 17
|supposed to complete bringup of the CPU
| (`cpuhp_state:1.done_up`) in cpuhp_thread_fun+0x108
|blocked on `wq_pool_attach_mutex`
| in workqueue_online_cpu+0x9e
|
xfs-conv/dm-0 task 745 |
holds the `wq_pool_attach_mutex` |
in worker_attach_to_pool+0x66 \
blocked on `task->migration_pending->done`|
completion in affine_move_task+0x10a/
~~~
crash> b 1429
PID: 1429 TASK: 99398000 CPU: 0 COMMAND: "cpuplugd"
#0 [997df970] __schedule+0x34c at 3089c424
#1 [997df9e0] schedule+0x7e at 3089cafe
#2 [997dfa20] schedule_timeout+0x26e at 308a1d8e
[inlined] do_wait_for_common
[inlined] __wait_for_common
#3 [997dfad8] wait_for_common+0x14a at 3089d902
[ret call] wait_for_completion+0x1a at 3089d96a
[inlined] wait_for_ap_thread <<< blocked on `cpuhp_state:1.done_up` completion
[ret call] __cpuhp_kick_ap+0x76 at 300c610e
#4 [997dfb58] cpuhp_kick_ap+0xc4 at 300c61dc
[inlined] bringup_wait_for_ap
[ret call] bringup_cpu+0xea at 300c6402
#5 [997dfba8] cpuhp_invoke_callback+0xcc at 300c4f14
#6 [997dfc40] _cpu_up+0x16a at 300c798a <<< holds the `cpu_hotplug_lock` for writing
#7 [997dfc98] do_cpu_up+0xc6 at 300c7b66
#8 [997dfcd8] cpu_subsys_online+0x58 at 305a0a00
#9 [997dfd28] device_online+0x9e at 30598e7e
#10 [997dfd68] online_store+0x88 at 30598f28
#11 [997dfda8] kernfs_fop_write+0xdc at 3040276c
#12 [997dfdf8] vfs_write+0xa8 at 30354760
#13 [997dfe58] ksys_write+0x62 at 30354a32
crash> cpuhp_cpu_state.state cpuhp_state:a | paste - -
[0]: 1aef424e0 state = CPUHP_ONLINE, # (195)
[1]: 1aef654e0 state = CPUHP_AP_WORKQUEUE_ONLINE, # (159)
crash> cpuhp_cpu_state.bringup,thread,done_up.done cpuhp_state:1 -d | paste - - - -
[1]: 1aef654e0 bringup = true, thread = 0x81134400, done_up.done = 0, <<<
crash> b 17
PID: 17 TASK: 81134400 CPU: 1 COMMAND: "cpuhp/1"
#0 [81143b68] __schedule+0x34c at 3089c424
#1 [81143bd8] schedule+0x7e at 3089cafe
#2 [81143c18] schedule_preempt_disabled+0x2a at 3089cfba
#3 [81143c30] __mutex_lock+0x320 at 3089df60
#4 [81143cb0] workqueue_online_cpu+0x9e at 300e847e <<< blocked on `wq_pool_attach_mutex`
#5 [81143d20] cpuhp_invoke_callback+0xcc at 300c4f14
#6 [81143db8] cpuhp_thread_fun+0x108 at 300c6848 <<< supposed to complete the bring-up of the CPU (`cpuhp_state:1.done_up`)
crash> b 745
PID: 745 TASK: 82359100 CPU: 0 COMMAND: "xfs-conv/dm-0"
#0 [8b4bfa20] __schedule+0x34c at 3089c424
#1 [8b4bfa90] schedule+0x7e at 3089cafe
#2 [8b4bfad0] schedule_timeout+0x26e at 308a1d8e
[inlined] do_wait_for_common
[inlined] __wait_for_common
#3 [8b4bfb88] wait_for_common+0x14a at 3089d902
[ret call] wait_for_completion+0x1a at 3089d96a
#4 [8b4bfc08] affine_move_task+0x10a at 300fb51a <<< blocked on `task->migration_pending->done` completion
#5 [8b4bfd08] __set_cpus_allowed_ptr+0x12e at 300fb926
[ret call] set_cpus_allowed_ptr+0xa at 300fba32
#6 [8b4bfd78] worker_attach_to_pool+0x66 at 300e1dae <<< holds the `wq_pool_attach_mutex`
#7 [8b4bfdc8] rescuer_thread+0x12c at 300e5bac
crash> rx 8b4bfea0
8b4bfea0: [863373c0:kmalloc-192]
crash> worker.task,rescue_wq 863373c0
task = 0x82359100,
rescue_wq = 0x8aa44400,
crash> list -s pool_workqueue.pool pool_workqueue.mayday_node -hO workqueue_struct.maydays 0x8aa44400 | paste - -
1fffff7f751900 pool = 0x1aef56a00,
crash> worker_pool.attrs 0x1aef56a00
attrs = 0x80088180,
crash> workqueue_attrs.cpumask[0].bits 0x80088180
cpumask[0].bits = {0x1, 0x0, ...
crash> cpumask.bits __cpu_active_mask
bits = {0x1, 0x0, ...
crash> cpumask.bits __cpu_online_mask
bits = {0x3, 0x0, ...
crash> task_struct.migration_pending,flags 0x82359100
migration_pending = 0x8b4bfce8,
flags = 0x4208060,
^ PF_KTHREAD
crash> pd distribute_cpu_mask_prev:0
per_cpu(distribute_cpu_mask_prev, 0) = 0
crash> set_affinity_pending.refs.refs.counter,arg,stop_pending,done.done 0x8b4bfce8 -d
refs.refs.counter = 1
arg = {
task = 0x82359100,
dest_cpu = 0,
pending = 0x8b4bfce8
}
stop_pending = 1,
done.done = 0,
~~~
In other words the `set_cpus_allowed_ptr()` is called from a worker thread which
tries to migrate. The worker pool is only allowed on CPU 0 and that was supposed
to be the destination as per the stack structure. In this case I thought it's OK
to leave the task on the old CPU and the Bill's testing scenario was successful
with the proposed patch. IIUC, it's exercising the hotplug due to load-balancing.
This was on RHEL 8.8.z kernel. I see upstream changed a bit so I'm not sure it's
still reproducible. Also, I'm not sure why this only happens on s390 and not on
x86. I imagine the CPU hotplug slightly differs? Anyways this seems to be timing
sensitive and the timing will differ greatly for sure.
> > Fixes: 9e81889c7648 ("sched: Fix affine_move_task() self-concurrency")
> > Cc: stable@xxxxxxxxxxxxxxx
> > Reported-by: Bill Peters <wpeters@xxxxxxxxx>
> > Tested-by: Bill Peters <wpeters@xxxxxxxxx>
> > Signed-off-by: Daniel Vacek <neelx@xxxxxxxxxx>
> > ---
> > kernel/sched/core.c | 13 +++++++++++--
> > 1 file changed, 11 insertions(+), 2 deletions(-)
> >
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index 9116bcc903467..d0ff5c611a1c8 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -3069,8 +3069,17 @@ static int affine_move_task(struct rq *rq, struct task_struct *p, struct rq_flag
> > preempt_disable();
> > task_rq_unlock(rq, p, rf);
> > if (!stop_pending) {
> > - stop_one_cpu_nowait(cpu_of(rq), migration_cpu_stop,
> > - &pending->arg, &pending->stop_work);
> > + stop_pending =
> > + stop_one_cpu_nowait(cpu_of(rq), migration_cpu_stop,
> > + &pending->arg, &pending->stop_work);
> > +
> > + if (!stop_pending) {
> > + rq = task_rq_lock(p, rf);
> > + pending->stop_pending = false;
> > + p->migration_pending = NULL;
> > + task_rq_unlock(rq, p, rf);
> > + complete_all(&pending->done);
> > + }
>
> This can leave the task @p on a now-illegal CPU; consider a task affined to
> CPUs 0-1, migrate_disable(); then affined to CPUs 2-3, then in
> migrate_enable() the dest_cpu is chosen as 3 but that's racing with it
> being brought down. The stop_one_cpu_nowait() fails, and we leave the task
> on CPUs 0-1.
>
> Issuing a redo of affine_move_task() with a different dest_cpu doesn't
> sound great, and while very unlikely that doesn't have forward progress
> guarantees.
>
> Unfortunately we can't hold the hotplug lock during the affinity change of
> migrate_enable(), as migrate_enable() isn't allowed to block.
>
> Now, the CPU selection in __set_cpus_allowed_ptr_locked() that is passed
> down to affine_move_task() relies on the active mask, which itself is
> cleared in sched_cpu_deactivate() and is followed by a
> synchronize_rcu().
>
> What if we made the affinity change of migrate_enable() an RCU read-side
> section? Then if a CPU X is observed as active in
> migrate_enable()->__set_cpus_allowed_ptr_locked()
> , then its' hotplug state cannot go lower than CPUHP_AP_ACTIVE until the task is
> migrated away.
>
> Something like the below. Thoughts?
> ---
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 929fce69f555e..c6d128711d1a9 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2450,8 +2450,11 @@ void migrate_enable(void)
> * __set_cpus_allowed_ptr(SCA_MIGRATE_ENABLE) doesn't schedule().
> */
> guard(preempt)();
> - if (p->cpus_ptr != &p->cpus_mask)
> + if (p->cpus_ptr != &p->cpus_mask) {
> + guard(rcu)();
> __set_cpus_allowed_ptr(p, &ac);
> + }
> +
> /*
> * Mustn't clear migration_disabled() until cpus_ptr points back at the
> * regular cpus_mask, otherwise things that race (eg.
With the above being said, I don't see any relation to `migrate_enable()` and
this change looks irrelevant in the context.
Any fresh ideas?
--nX
