Re: severe proc dentry lock contention

[ebiederm@xmission.com (Eric W. Biederman)]

Junxiao Bi <junxiao.bi@oracle.com> writes:

> 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.

Assuming this works we can remove the hacking part of always doing
this by only doing this if SIGNAL_GROUP_EXIT when we know this
thundering herd will appear.

We still need to do something with the list however.

If your testing works out performance wise I will figure out what
we need to make the hack generale and safe.

Eric



> 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);
>>