Re: [PATCH] sched/schedutil: Fix deadlock between cpuset and cpu hotplug when using schedutil

[Xuewen Yan]

Hi all

This deadlock is inevitable, any comment?

On Tue, Jul 5, 2022 at 11:03 PM Xuewen Yan <xuewen.yan@xxxxxxxxxx> wrote:
>
> There is a deadlock between cpu_hotplug_lock sem and cgroup_threadgroup_rwsem.
> And both cpu online and cpu offline scenarios will cause deadlock:
>
> [1] cpu online:
>
> 1. first, thread-A get the cgroup_threadgroup_rwsem and waiting foe the cpu_hotplug_lock:
>
>   [ffffffc00f9c3850] __switch_to at ffffffc0081229d4
>   [ffffffc00f9c38b0] __schedule at ffffffc009c824f8
>   [ffffffc00f9c3910] schedule at ffffffc009c82b50
>   [ffffffc00f9c3970] percpu_rwsem_wait at ffffffc00828fbcc
>   [ffffffc00f9c39b0] __percpu_down_read at ffffffc008290180     <<< try to get cpu_hotplug_lock
>   [ffffffc00f9c39e0] cpus_read_lock at ffffffc0081dade8
>   [ffffffc00f9c3a20] cpuset_attach at ffffffc008366ed4
>   [ffffffc00f9c3a90] cgroup_migrate_execute at ffffffc00834f7d8
>   [ffffffc00f9c3b60] cgroup_attach_task at ffffffc0083539b8
>   [ffffffc00f9c3bd0] __cgroup1_procs_write at ffffffc00835f6ac   <<< get the cgroup_threadgroup_rwsem
>   [ffffffc00f9c3c30] cgroup1_tasks_write at ffffffc00835f018
>   [ffffffc00f9c3c60] cgroup_file_write at ffffffc008348b04
>   [ffffffc00f9c3c90] kernfs_fop_write_iter at ffffffc0087544d8
>   [ffffffc00f9c3d50] vfs_write at ffffffc008607a8c
>   [ffffffc00f9c3db0] ksys_write at ffffffc0086076d8
>   [ffffffc00f9c3df0] __arm64_sys_write at ffffffc008607640
>   [ffffffc00f9c3e10] invoke_syscall at ffffffc00813dccc
>   [ffffffc00f9c3e30] el0_svc_common at ffffffc00813dbe4
>   [ffffffc00f9c3e70] do_el0_svc at ffffffc00813dac8
>   [ffffffc00f9c3e80] el0_svc at ffffffc0098836c8
>   [ffffffc00f9c3ea0] el0t_64_sync_handler at ffffffc00988363c
>   [ffffffc00f9c3fe0] el0t_64_sync at ffffffc008091e44
>
> 2. Thread-B get the cpu_hotplug_lock and waiting for cpuhp/x:
>
>   [ffffffc01245b740] __switch_to at ffffffc0081229d4
>   [ffffffc01245b7a0] __schedule at ffffffc009c824f8
>   [ffffffc01245b800] schedule at ffffffc009c82b50
>   [ffffffc01245b880] schedule_timeout at ffffffc009c8a144
>   [ffffffc01245b8e0] wait_for_common at ffffffc009c83dac     <<< waiting for cpuhp/x complete
>   [ffffffc01245b940] cpuhp_kick_ap at ffffffc0081dc8ac       <<< wake up cpuhp/x
>   [ffffffc01245b980] bringup_cpu at ffffffc0081db7f0
>   [ffffffc01245ba00] cpuhp_invoke_callback at ffffffc0081dc000
>   [ffffffc01245ba60] cpuhp_up_callbacks at ffffffc0081dd9b0
>   [ffffffc01245bac0] _cpu_up at ffffffc0081dd844            <<< percpu_down_write(&cpu_hotplug_lock)
>   [ffffffc01245bb40] cpu_up at ffffffc0081dd100
>   [ffffffc01245bb80] cpu_subsys_online at ffffffc008d8ebe4
>   [ffffffc01245bb90] device_online at ffffffc008d77dec
>   [ffffffc01245bbd0] online_store at ffffffc008d77bb8
>   [ffffffc01245bc30] dev_attr_store at ffffffc008d7c8b8
>   [ffffffc01245bc60] sysfs_kf_write at ffffffc008757894
>   [ffffffc01245bc90] kernfs_fop_write_iter at ffffffc0087544d8
>   [ffffffc01245bd50] vfs_write at ffffffc008607a8c
>   [ffffffc01245bdb0] ksys_write at ffffffc0086076d8
>   [ffffffc01245bdf0] __arm64_sys_write at ffffffc008607640
>   [ffffffc01245be10] invoke_syscall at ffffffc00813dccc
>   [ffffffc01245be30] el0_svc_common at ffffffc00813dc14
>   [ffffffc01245be70] do_el0_svc at ffffffc00813dac8
>   [ffffffc01245be80] el0_svc at ffffffc0098836c8
>   [ffffffc01245bea0] el0t_64_sync_handler at ffffffc00988363c
>   [ffffffc01245bfe0] el0t_64_sync at ffffffc008091e44
>
> 3. cpuhp/x would call cpufreq_online and the schedutil need create kthread, as a result it need wait for kthreadd:
>
>   [ffffffc00b653860] __switch_to at ffffffc0081229d4
>   [ffffffc00b6538c0] __schedule at ffffffc009c824f8
>   [ffffffc00b653920] schedule at ffffffc009c82b50
>   [ffffffc00b6539a0] schedule_timeout at ffffffc009c8a144
>   [ffffffc00b653a00] wait_for_common at ffffffc009c83dac     <<< wait for kthreadd
>   [ffffffc00b653ad0] __kthread_create_on_node at ffffffc0082296b8
>   [ffffffc00b653b90] kthread_create_on_node at ffffffc008229a58
>   [ffffffc00b653bb0] sugov_init at ffffffc001d01414
>   [ffffffc00b653c00] cpufreq_init_governor at ffffffc009179a48
>   [ffffffc00b653c50] cpufreq_set_policy at ffffffc009179360
>   [ffffffc00b653cb0] cpufreq_online at ffffffc00917c3e4
>   [ffffffc00b653d10] cpuhp_cpufreq_online at ffffffc00917ee3c
>   [ffffffc00b653d70] cpuhp_invoke_callback at ffffffc0081dc000
>   [ffffffc00b653dd0] cpuhp_thread_fun at ffffffc0081df47c
>   [ffffffc00b653e10] smpboot_thread_fn at ffffffc008234718
>   [ffffffc00b653e70] kthread at ffffffc00822ac84
>
> 4. the kthreadd is trying to get cgroup_threadgroup_rwsem...
>
>   [ffffffc008073a70] __switch_to at ffffffc0081229d4
>   [ffffffc008073ad0] __schedule at ffffffc009c824f8
>   [ffffffc008073b30] schedule at ffffffc009c82b50
>   [ffffffc008073b90] percpu_rwsem_wait at ffffffc00828fbcc  <<< cgroup_threadgroup_rwsem
>   [ffffffc008073bd0] __percpu_down_read at ffffffc008290180
>   [ffffffc008073c00] cgroup_css_set_fork at ffffffc008357ecc
>   [ffffffc008073c60] cgroup_can_fork at ffffffc008357bb4
>   [ffffffc008073d00] copy_process at ffffffc0081d18c4
>   [ffffffc008073d90] kernel_clone at ffffffc0081d088c
>   [ffffffc008073e50] kthreadd at ffffffc00822a8e4
>
> Finally, the Thread-A and Thread-B would be blockd forever.
>
> [2]cpu_offline:
>
> 1. first, thread-A get the cgroup_threadgroup_rwsem and waiting foe the cpu_hotplug_lock:
>
>   [ffffffc00f9c3850] __switch_to at ffffffc0081229d4
>   [ffffffc00f9c38b0] __schedule at ffffffc009c824f8
>   [ffffffc00f9c3910] schedule at ffffffc009c82b50
>   [ffffffc00f9c3970] percpu_rwsem_wait at ffffffc00828fbcc
>   [ffffffc00f9c39b0] __percpu_down_read at ffffffc008290180     <<< try to get cpu_hotplug_lock
>   [ffffffc00f9c39e0] cpus_read_lock at ffffffc0081dade8
>   [ffffffc00f9c3a20] cpuset_attach at ffffffc008366ed4
>   [ffffffc00f9c3a90] cgroup_migrate_execute at ffffffc00834f7d8
>   [ffffffc00f9c3b60] cgroup_attach_task at ffffffc0083539b8
>   [ffffffc00f9c3bd0] __cgroup1_procs_write at ffffffc00835f6ac   <<< get the cgroup_threadgroup_rwsem
>   [ffffffc00f9c3c30] cgroup1_tasks_write at ffffffc00835f018
>   [ffffffc00f9c3c60] cgroup_file_write at ffffffc008348b04
>   [ffffffc00f9c3c90] kernfs_fop_write_iter at ffffffc0087544d8
>   [ffffffc00f9c3d50] vfs_write at ffffffc008607a8c
>   [ffffffc00f9c3db0] ksys_write at ffffffc0086076d8
>   [ffffffc00f9c3df0] __arm64_sys_write at ffffffc008607640
>   [ffffffc00f9c3e10] invoke_syscall at ffffffc00813dccc
>   [ffffffc00f9c3e30] el0_svc_common at ffffffc00813dbe4
>   [ffffffc00f9c3e70] do_el0_svc at ffffffc00813dac8
>   [ffffffc00f9c3e80] el0_svc at ffffffc0098836c8
>   [ffffffc00f9c3ea0] el0t_64_sync_handler at ffffffc00988363c
>   [ffffffc00f9c3fe0] el0t_64_sync at ffffffc008091e44
>
> 2. Thread-B get the cpu_hotplug_lock and waiting for cpuhp/x:
>
>   [ffffffc01245b740] __switch_to at ffffffc0081229d4
>   [ffffffc01245b7a0] __schedule at ffffffc009c824f8
>   [ffffffc01245b800] schedule at ffffffc009c82b50
>   [ffffffc01245b880] schedule_timeout at ffffffc009c8a144
>   [ffffffc01245b8e0] wait_for_common at ffffffc009c83dac     <<< waiting for cpuhp/x complete
>   [ffffffc01245b940] cpuhp_kick_ap at ffffffc0081dc8ac       <<< wake up cpuhp/x
>   [ffffffc01245b980] bringup_cpu at ffffffc0081db7f0
>   [ffffffc01245ba00] cpuhp_invoke_callback at ffffffc0081dc000
>   [ffffffc01245ba60] cpuhp_up_callbacks at ffffffc0081dd9b0
>   [ffffffc01245bac0] _cpu_up at ffffffc0081dd844            <<< percpu_down_write(&cpu_hotplug_lock)
>   [ffffffc01245bb40] cpu_up at ffffffc0081dd100
>   [ffffffc01245bb80] cpu_subsys_online at ffffffc008d8ebe4
>   [ffffffc01245bb90] device_online at ffffffc008d77dec
>   [ffffffc01245bbd0] online_store at ffffffc008d77bb8
>   [ffffffc01245bc30] dev_attr_store at ffffffc008d7c8b8
>   [ffffffc01245bc60] sysfs_kf_write at ffffffc008757894
>   [ffffffc01245bc90] kernfs_fop_write_iter at ffffffc0087544d8
>   [ffffffc01245bd50] vfs_write at ffffffc008607a8c
>   [ffffffc01245bdb0] ksys_write at ffffffc0086076d8
>   [ffffffc01245bdf0] __arm64_sys_write at ffffffc008607640
>   [ffffffc01245be10] invoke_syscall at ffffffc00813dccc
>   [ffffffc01245be30] el0_svc_common at ffffffc00813dc14
>   [ffffffc01245be70] do_el0_svc at ffffffc00813dac8
>   [ffffffc01245be80] el0_svc at ffffffc0098836c8
>   [ffffffc01245bea0] el0t_64_sync_handler at ffffffc00988363c
>   [ffffffc01245bfe0] el0t_64_sync at ffffffc008091e44
>
> 3.cpuhp/x would call cpufreq_offline and schedutil need stop_kthread:
>
>   [ffffffc00b683a60] __switch_to at ffffffc0081229d4
>   [ffffffc00b683ac0] __schedule at ffffffc009c824f8
>   [ffffffc00b683b20] schedule at ffffffc009c82b50
>   [ffffffc00b683ba0] schedule_timeout at ffffffc009c8a144
>   [ffffffc00b683c00] wait_for_common at ffffffc009c83dac <<< waiting for sugov complete
>   [ffffffc00b683c60] kthread_stop at ffffffc008228128   <<< wakeup sugov
>   [ffffffc00b683c90] sugov_exit at ffffffc001d016a8
>   [ffffffc00b683cc0] cpufreq_offline at ffffffc00917b4b8
>   [ffffffc00b683d10] cpuhp_cpufreq_offline at ffffffc00917ee70
>   [ffffffc00b683d70] cpuhp_invoke_callback at ffffffc0081dc000
>   [ffffffc00b683dd0] cpuhp_thread_fun at ffffffc0081df47c
>   [ffffffc00b683e10] smpboot_thread_fn at ffffffc008234718
>   [ffffffc00b683e70] kthread at ffffffc00822ac84
>
> 4. the sugov thread is waiting cgroup_threadgroup_rwsem:
>
>   [ffffffc01258bc10] __switch_to at ffffffc0081229d4
>   [ffffffc01258bc70] __schedule at ffffffc009c824f8
>   [ffffffc01258bcd0] schedule at ffffffc009c82b50
>   [ffffffc01258bd30] percpu_rwsem_wait at ffffffc00828fbcc  <<< wait for cgroup_threadgroup_rwsem
>   [ffffffc01258bd70] __percpu_down_read at ffffffc008290180
>   [ffffffc01258bdb0] exit_signals at ffffffc0082074c8
>   [ffffffc01258be10] do_exit at ffffffc0081e5130
>   [ffffffc01258be70] kthread at ffffffc00822ac8c
>
> Finally, the Thread-A and Thread-B would be blockd forever.
>
> Combining the above two situations, the reason why Thread-A and Thread-B are blocked is because cpuhp/x is blocked.
> If we can solve the problem that cpuhp is not blocked, Thread-A and Thread-B can run normally.
> So we can let schedutil do not create or destory thread, so that the cpuhp/x would not be blocked.
>
> So init the sugov thread just once to prevent the above deadlock.
>
> Signed-off-by: Ke Wang <ke.wang@xxxxxxxxxx>
> Signed-off-by: Xuewen Yan <xuewen.yan@xxxxxxxxxx>
> ---
>  kernel/sched/cpufreq_schedutil.c | 24 +++++++++++++++++++++++-
>  1 file changed, 23 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> index 285ad51caf0f..42d04a1eac20 100644
> --- a/kernel/sched/cpufreq_schedutil.c
> +++ b/kernel/sched/cpufreq_schedutil.c
> @@ -8,6 +8,8 @@
>
>  #define IOWAIT_BOOST_MIN       (SCHED_CAPACITY_SCALE / 8)
>
> +static LIST_HEAD(sugov_list);
> +
>  struct sugov_tunables {
>         struct gov_attr_set     attr_set;
>         unsigned int            rate_limit_us;
> @@ -15,6 +17,7 @@ struct sugov_tunables {
>
>  struct sugov_policy {
>         struct cpufreq_policy   *policy;
> +       struct list_head        sg_policy_list;
>
>         struct sugov_tunables   *tunables;
>         struct list_head        tunables_hook;
> @@ -659,6 +662,7 @@ static int sugov_init(struct cpufreq_policy *policy)
>         struct sugov_policy *sg_policy;
>         struct sugov_tunables *tunables;
>         int ret = 0;
> +       int sugov_init = 0;
>
>         /* State should be equivalent to EXIT */
>         if (policy->governor_data)
> @@ -666,6 +670,13 @@ static int sugov_init(struct cpufreq_policy *policy)
>
>         cpufreq_enable_fast_switch(policy);
>
> +       list_for_each_entry(sg_policy, &sugov_list, sg_policy_list) {
> +               if (sg_policy->policy == policy) {
> +                       sugov_init = 1;
> +                       goto tunables_lock;
> +               }
> +       }
> +
>         sg_policy = sugov_policy_alloc(policy);
>         if (!sg_policy) {
>                 ret = -ENOMEM;
> @@ -676,6 +687,10 @@ static int sugov_init(struct cpufreq_policy *policy)
>         if (ret)
>                 goto free_sg_policy;
>
> +       list_add(&sg_policy->sg_policy_list, &sugov_list);
> +
> +tunables_lock:
> +
>         mutex_lock(&global_tunables_lock);
>
>         if (global_tunables) {
> @@ -717,7 +732,8 @@ static int sugov_init(struct cpufreq_policy *policy)
>         sugov_clear_global_tunables();
>
>  stop_kthread:
> -       sugov_kthread_stop(sg_policy);
> +       if (!sugov_init)
> +               sugov_kthread_stop(sg_policy);
>         mutex_unlock(&global_tunables_lock);
>
>  free_sg_policy:
> @@ -745,8 +761,14 @@ static void sugov_exit(struct cpufreq_policy *policy)
>
>         mutex_unlock(&global_tunables_lock);
>
> +       list_for_each_entry(sg_policy, &sugov_list, sg_policy_list) {
> +               if (sg_policy->policy == policy) {
> +                       goto out;
> +               }
> +       }
>         sugov_kthread_stop(sg_policy);
>         sugov_policy_free(sg_policy);
> +out:
>         cpufreq_disable_fast_switch(policy);
>  }
>
> --
> 2.25.1
>

Thanks！
BR