Re: severe proc dentry lock contention

[Junxiao Bi <junxiao.bi@oracle.com>]

On 6/18/20 5:02 PM, ebiederm@xmission.com wrote:

> Matthew Wilcox <willy@infradead.org> writes:
>
>> On Thu, Jun 18, 2020 at 03:17:33PM -0700, Junxiao Bi wrote:
>>> When debugging some performance issue, i found that thousands of threads
>>> exit around same time could cause a severe spin lock contention on proc
>>> dentry "/proc/$parent_process_pid/task/", that's because threads needs to
>>> clean up their pid file from that dir when exit. Check the following
>>> standalone test case that simulated the case and perf top result on v5.7
>>> kernel. Any idea on how to fix this?
>> Thanks, Junxiao.
>>
>> We've looked at a few different ways of fixing this problem.
>>
>> Even though the contention is within the dcache, it seems like a usecase
>> that the dcache shouldn't be optimised for -- generally we do not have
>> hundreds of CPUs removing dentries from a single directory in parallel.
>>
>> We could fix this within procfs.  We don't have a great patch yet, but
>> the current approach we're looking at allows only one thread at a time
>> to call dput() on any /proc/*/task directory.
>>
>> We could also look at fixing this within the scheduler.  Only allowing
>> one CPU to run the threads of an exiting process would fix this particular
>> problem, but might have other consequences.
>>
>> I was hoping that 7bc3e6e55acf would fix this, but that patch is in 5.7,
>> so that hope is ruled out.
> Does anyone know if problem new in v5.7?  I am wondering if I introduced
> this problem when I refactored the code or if I simply churned the code
> but the issue remains effectively the same.
It's not new issue, we see it in old kernel like v4.14
>
> Can you try only flushing entries when the last thread of the process is
> reaped?  I think in practice we would want to be a little more
> sophisticated but it is a good test case to see if it solves the issue.

Thank you. i will try and let you know.

Thanks,

Junxiao.

>
> diff --git a/kernel/exit.c b/kernel/exit.c
> index cebae77a9664..d56e4eb60bdd 100644
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -152,7 +152,7 @@ void put_task_struct_rcu_user(struct task_struct *task)
>   void release_task(struct task_struct *p)
>   {
>   	struct task_struct *leader;
> -	struct pid *thread_pid;
> +	struct pid *thread_pid = NULL;
>   	int zap_leader;
>   repeat:
>   	/* don't need to get the RCU readlock here - the process is dead and
> @@ -165,7 +165,8 @@ void release_task(struct task_struct *p)
>   
>   	write_lock_irq(&tasklist_lock);
>   	ptrace_release_task(p);
> -	thread_pid = get_pid(p->thread_pid);
> +	if (p == p->group_leader)
> +		thread_pid = get_pid(p->thread_pid);
>   	__exit_signal(p);
>   
>   	/*
> @@ -188,8 +189,10 @@ void release_task(struct task_struct *p)
>   	}
>   
>   	write_unlock_irq(&tasklist_lock);
> -	proc_flush_pid(thread_pid);
> -	put_pid(thread_pid);
> +	if (thread_pid) {
> +		proc_flush_pid(thread_pid);
> +		put_pid(thread_pid);
> +	}
>   	release_thread(p);
>   	put_task_struct_rcu_user(p);
>