Hi Valentin,
On Mon, Mar 18, 2024 at 6:34 PM Valentin Schneider vschneid@redhat.com 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@vger.kernel.org Reported-by: Bill Peters wpeters@atpco.net Tested-by: Bill Peters wpeters@atpco.net Signed-off-by: Daniel Vacek neelx@redhat.com
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)();- }
- /*
- 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