Messages in this thread | | | Subject | Re: severe proc dentry lock contention | From | Junxiao Bi <> | Date | Thu, 18 Jun 2020 17:27:43 -0700 |
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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); >
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