Re: [PATCH] cpuset: adjust the lock sequence when rebuilding the sched domains.

[Juri Lelli]

Hi,

On 05/09/19 10:18, Chunyan Zhang wrote:
> From: Vincent Wang <vincent.wang@xxxxxxxxxx>
> 
> A deadlock issue is found when executing a cpu hotplug stress test on
> android phones with cpuset and scheduil enabled.
> 
> When CPUx is plugged out, the hotplug thread that calls cpu_down()
> will hold cpu_hotplug_lock and wait the thread cpuhp/x to finish
> hotplug. If the core is the last one in a cluster, cpuhp/x have to
> call cpuhp_cpufreq_offline() and the kernel thread sugov need to exit
> for schedutil governor. The exit of sugov need to hold
> cgroup_threadgroup_rwsem in exit_signals(). For example:
> 
> PID: 150    TASK: ffffffc0b9cad080  CPU: 0   COMMAND: "sprdhotplug"
>  #0 [ffffff8009fcb9d0] __switch_to at ffffff80080858f0
>  #1 [ffffff8009fcb9f0] __schedule at ffffff80089f185c
>  #2 [ffffff8009fcba80] schedule at ffffff80089f1b84
>  #3 [ffffff8009fcbaa0] schedule_timeout at ffffff80089f5124
>  #4 [ffffff8009fcbb40] wait_for_common at ffffff80089f2944
>  #5 [ffffff8009fcbbe0] wait_for_completion at ffffff80089f29a4
>  #6 [ffffff8009fcbc00] __cpuhp_kick_ap at ffffff80080ab030
>  #7 [ffffff8009fcbc20] cpuhp_kick_ap_work at ffffff80080ab154
>  #8 [ffffff8009fcbc70] _cpu_down at ffffff80089ee19c
>  #9 [ffffff8009fcbcd0] cpu_down at ffffff80080ac144
> 
> PID: 26     TASK: ffffffc0bbe22080  CPU: 3   COMMAND: "cpuhp/3"
>  #0 [ffffff8009693a30] __switch_to at ffffff80080858f0
>  #1 [ffffff8009693a50] __schedule at ffffff80089f185c
>  #2 [ffffff8009693ae0] schedule at ffffff80089f1b84
>  #3 [ffffff8009693b00] schedule_timeout at ffffff80089f5124
>  #4 [ffffff8009693ba0] wait_for_common at ffffff80089f2944
>  #5 [ffffff8009693c40] wait_for_completion at ffffff80089f29a4
>  #6 [ffffff8009693c60] kthread_stop at ffffff80080ccd2c
>  #7 [ffffff8009693c90] sugov_exit at ffffff8008102134
>  #8 [ffffff8009693cc0] cpufreq_exit_governor at ffffff80086c03bc
>  #9 [ffffff8009693ce0] cpufreq_offline at ffffff80086c0634
> 
> PID: 13819  TASK: ffffffc0affb6080  CPU: 0   COMMAND: "sugov:3"
>  #0 [ffffff800ee73c30] __switch_to at ffffff80080858f0
>  #1 [ffffff800ee73c50] __schedule at ffffff80089f185c
>  #2 [ffffff800ee73ce0] schedule at ffffff80089f1b84
>  #3 [ffffff800ee73d00] rwsem_down_read_failed at ffffff80089f49d0
>  #4 [ffffff800ee73d80] __percpu_down_read at ffffff8008102ebc
>  #5 [ffffff800ee73da0] exit_signals at ffffff80080bbd24
>  #6 [ffffff800ee73de0] do_exit at ffffff80080ae65c
>  #7 [ffffff800ee73e60] kthread at ffffff80080cc550
> 
> Sometimes cgroup_threadgroup_rwsem is hold by another thread, for
> example Binder:681_2 on android, it wants to hold cpuset_mutex:
> 
> PID: 732    TASK: ffffffc09668b080  CPU: 2   COMMAND: "Binder:681_2"
>  #0 [ffffff800cb7b8c0] __switch_to at ffffff80080858f0
>  #1 [ffffff800cb7b8e0] __schedule at ffffff80089f185c
>  #2 [ffffff800cb7b970] schedule at ffffff80089f1b84
>  #3 [ffffff800cb7b990] schedule_preempt_disabled at ffffff80089f205c
>  #4 [ffffff800cb7b9a0] __mutex_lock at ffffff80089f3118
>  #5 [ffffff800cb7ba40] __mutex_lock_slowpath at ffffff80089f3230
>  #6 [ffffff800cb7ba60] mutex_lock at ffffff80089f3278
>  #7 [ffffff800cb7ba80] cpuset_can_attach at ffffff8008152f84
>  #8 [ffffff800cb7bae0] cgroup_migrate_execute at ffffff800814ada8
> 
> On android, a thread kworker/3:0 will hold cpuset_mutex in
> rebuild_sched_domains() and want to hold cpu_hotplug_lock which
> is already hold by the hotplug thread.
> 
> PID: 4847   TASK: ffffffc031a6a080  CPU: 3   COMMAND: "kworker/3:0"
>  #0 [ffffff8016fd3ad0] __switch_to at ffffff80080858f0
>  #1 [ffffff8016fd3af0] __schedule at ffffff80089f185c
>  #2 [ffffff8016fd3b80] schedule at ffffff80089f1b84
>  #3 [ffffff8016fd3ba0] rwsem_down_read_failed at ffffff80089f49d0
>  #4 [ffffff8016fd3c20] __percpu_down_read at ffffff8008102ebc
>  #5 [ffffff8016fd3c40] cpus_read_lock at ffffff80080aa59c
>  #6 [ffffff8016fd3c50] rebuild_sched_domains_locked at ffffff80081522a8
> 
> In order to fix the deadlock, this patch will adjust the lock sequence
> when rebuilding sched domains. After stress tests, it works well.
> 
> Signed-off-by: Vincent Wang <vincent.wang@xxxxxxxxxx>
> Signed-off-by: Chunyan Zhang <zhang.lyra@xxxxxxxxx>
> ---
>  kernel/cgroup/cpuset.c | 9 ++++-----
>  1 file changed, 4 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 5aa37531ce76..ef10d276da22 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -912,7 +912,6 @@ static void rebuild_sched_domains_locked(void)
>  	int ndoms;
>  
>  	lockdep_assert_held(&cpuset_mutex);
> -	get_online_cpus();
>  
>  	/*
>  	 * We have raced with CPU hotplug. Don't do anything to avoid
> @@ -921,19 +920,17 @@ static void rebuild_sched_domains_locked(void)
>  	 */
>  	if (!top_cpuset.nr_subparts_cpus &&
>  	    !cpumask_equal(top_cpuset.effective_cpus, cpu_active_mask))
> -		goto out;
> +		return;
>  
>  	if (top_cpuset.nr_subparts_cpus &&
>  	   !cpumask_subset(top_cpuset.effective_cpus, cpu_active_mask))
> -		goto out;
> +		return;
>  
>  	/* Generate domain masks and attrs */
>  	ndoms = generate_sched_domains(&doms, &attr);
>  
>  	/* Have scheduler rebuild the domains */
>  	partition_sched_domains(ndoms, doms, attr);
> -out:
> -	put_online_cpus();
>  }
>  #else /* !CONFIG_SMP */
>  static void rebuild_sched_domains_locked(void)
> @@ -943,9 +940,11 @@ static void rebuild_sched_domains_locked(void)
>  
>  void rebuild_sched_domains(void)
>  {
> +	get_online_cpus();
>  	mutex_lock(&cpuset_mutex);
>  	rebuild_sched_domains_locked();
>  	mutex_unlock(&cpuset_mutex);
> +	put_online_cpus();
>  }

This looks a subset of d74b27d63a8b ("cgroup/cpuset: Change cpuset_rwsem
and hotplug lock order") from

git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git sched/core

right?

Cc-ing Peter for reference.

Best,

Juri