Re: [PATCH] proc: Avoid a thundering herd of threads freeing proc dentries

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

Hi Eric,

The patch didn't improve lock contention.

    PerfTop:   48925 irqs/sec  kernel:95.6%  exact: 100.0% lost: 0/0 
drop: 0/0 [4000Hz cycles],  (all, 104 CPUs)
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 


     69.66%  [kernel]                                        [k] 
native_queued_spin_lock_slowpath
      1.93%  [kernel]                                        [k] 
_raw_spin_lock
      1.24%  [kernel]                                        [k] 
page_counter_cancel
      0.70%  [kernel]                                        [k] 
do_syscall_64
      0.62%  [kernel]                                        [k] 
find_idlest_group.isra.96
      0.57%  [kernel]                                        [k] 
queued_write_lock_slowpath
      0.56%  [kernel]                                        [k] d_walk
      0.45%  [kernel]                                        [k] 
clear_page_erms
      0.44%  [kernel]                                        [k] 
syscall_return_via_sysret
      0.40%  [kernel]                                        [k] 
entry_SYSCALL_64
      0.38%  [kernel]                                        [k] 
refcount_dec_not_one
      0.37%  [kernel]                                        [k] 
propagate_protected_usage
      0.33%  [kernel]                                        [k] 
unmap_page_range
      0.33%  [kernel]                                        [k] 
select_collect
      0.32%  [kernel]                                        [k] memcpy_erms
      0.30%  [kernel]                                        [k] 
proc_task_readdir
      0.27%  [kernel]                                        [k] 
_raw_spin_lock_irqsave

Thanks,

Junxiao.

On 6/19/20 7:09 AM, ebiederm@xmission.com wrote:
> Junxiao Bi <junxiao.bi@oracle.com> reported:
>> 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.
> Matthew Wilcox <willy@infradead.org> reported:
>> We've looked at a few different ways of fixing this problem.
> The flushing of the proc dentries from the dcache is an optmization,
> and is not necessary for correctness.  Eventually cache pressure will
> cause the dentries to be freed even if no flushing happens.  Some
> light testing when I refactored the proc flushg[1] indicated that at
> least the memory footprint is easily measurable.
>
> An optimization that causes a performance problem due to a thundering
> herd of threads is no real optimization.
>
> Modify the code to only flush the /proc/<tgid>/ directory when all
> threads in a process are killed at once.  This continues to flush
> practically everything when the process is reaped as the threads live
> under /proc/<tgid>/task/<tid>.
>
> There is a rare possibility that a debugger will access /proc/<tid>/,
> which this change will no longer flush, but I believe such accesses
> are sufficiently rare to not be observed in practice.
>
> [1] 7bc3e6e55acf ("proc: Use a list of inodes to flush from proc")
> Link: https://lkml.kernel.org/r/54091fc0-ca46-2186-97a8-d1f3c4f3877b@oracle.com
> Reported-by: Masahiro Yamada <masahiroy@kernel.org>
> Reported-by: Matthew Wilcox <willy@infradead.org>
> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
> ---
>
> I am still waiting for word on how this affects performance, but this is
> a clean version that should avoid the thundering herd problem in
> general.
>
>
>   kernel/exit.c | 19 +++++++++++++++----
>   1 file changed, 15 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/exit.c b/kernel/exit.c
> index cebae77a9664..567354550d62 100644
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -151,8 +151,8 @@ void put_task_struct_rcu_user(struct task_struct *task)
>   
>   void release_task(struct task_struct *p)
>   {
> +	struct pid *flush_pid = NULL;
>   	struct task_struct *leader;
> -	struct pid *thread_pid;
>   	int zap_leader;
>   repeat:
>   	/* don't need to get the RCU readlock here - the process is dead and
> @@ -165,7 +165,16 @@ void release_task(struct task_struct *p)
>   
>   	write_lock_irq(&tasklist_lock);
>   	ptrace_release_task(p);
> -	thread_pid = get_pid(p->thread_pid);
> +
> +	/*
> +	 * When all of the threads are exiting wait until the end
> +	 * and flush everything.
> +	 */
> +	if (thread_group_leader(p))
> +		flush_pid = get_pid(task_tgid(p));
> +	else if (!(p->signal->flags & SIGNAL_GROUP_EXIT))
> +		flush_pid = get_pid(task_pid(p));
> +
>   	__exit_signal(p);
>   
>   	/*
> @@ -188,8 +197,10 @@ void release_task(struct task_struct *p)
>   	}
>   
>   	write_unlock_irq(&tasklist_lock);
> -	proc_flush_pid(thread_pid);
> -	put_pid(thread_pid);
> +	if (flush_pid) {
> +		proc_flush_pid(flush_pid);
> +		put_pid(flush_pid);
> +	}
>   	release_thread(p);
>   	put_task_struct_rcu_user(p);
>