Re: severe proc dentry lock contention

[Eric W. Biederman]

Matthew Wilcox <willy@xxxxxxxxxxxxx> 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.

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.

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);