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Message-ID: <caa9adf6-e1bb-167b-6f59-d17fd587d4fa@oracle.com>
Date: Fri, 19 Jun 2020 08:56:21 -0700
From: Junxiao Bi <junxiao.bi@...cle.com>
To: "Eric W. Biederman" <ebiederm@...ssion.com>
Cc: Matthew Wilcox <willy@...radead.org>, linux-kernel@...r.kernel.org,
linux-fsdevel@...r.kernel.org,
Matthew Wilcox <matthew.wilcox@...cle.com>,
Srinivas Eeda <SRINIVAS.EEDA@...cle.com>,
"joe.jin@...cle.com" <joe.jin@...cle.com>,
Wengang Wang <wen.gang.wang@...cle.com>
Subject: Re: [PATCH] proc: Avoid a thundering herd of threads freeing proc
dentries
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@...ssion.com wrote:
> Junxiao Bi <junxiao.bi@...cle.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@...radead.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@...nel.org>
> Reported-by: Matthew Wilcox <willy@...radead.org>
> Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.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);
>
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