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From: Masahiro Yamada <masahiroy@kernel.org>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Junxiao Bi <junxiao.bi@oracle.com>,
	Matthew Wilcox <willy@infradead.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Linux FS-devel Mailing List <linux-fsdevel@vger.kernel.org>,
	Matthew Wilcox <matthew.wilcox@oracle.com>,
	Srinivas Eeda <SRINIVAS.EEDA@oracle.com>,
	"joe.jin@oracle.com" <joe.jin@oracle.com>
Subject: Re: [PATCH] proc: Avoid a thundering herd of threads freeing proc dentries
Date: Mon, 22 Jun 2020 14:33:39 +0900	[thread overview]
Message-ID: <CAK7LNARr4aWTUqcS5TGdQ-7C_u=PFQVMMuakQ2Oro3-43fYu9w@mail.gmail.com> (raw)
In-Reply-To: <87bllf87ve.fsf_-_@x220.int.ebiederm.org>

On Fri, Jun 19, 2020 at 11:14 PM Eric W. Biederman
<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>

I did not report this.





> Reported-by: Matthew Wilcox <willy@infradead.org>
> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
> ---


-- 
Best Regards
Masahiro Yamada

  parent reply	other threads:[~2020-06-22  5:38 UTC|newest]

Thread overview: 20+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-06-18 22:17 severe proc dentry lock contention Junxiao Bi
2020-06-18 23:39 ` Matthew Wilcox
2020-06-19  0:02   ` Eric W. Biederman
2020-06-19  0:27     ` Junxiao Bi
2020-06-19  3:30       ` Eric W. Biederman
2020-06-19 14:09       ` [PATCH] proc: Avoid a thundering herd of threads freeing proc dentries Eric W. Biederman
2020-06-19 15:56         ` Junxiao Bi
2020-06-19 17:24           ` Eric W. Biederman
2020-06-19 21:56             ` Junxiao Bi
2020-06-19 22:42               ` Eric W. Biederman
2020-06-20 16:27                 ` Matthew Wilcox
2020-06-22  5:15                   ` Junxiao Bi
2020-06-22 15:20                     ` Eric W. Biederman
2020-06-22 15:48                       ` willy
2020-08-17 12:19                         ` Eric W. Biederman
2020-06-22 17:16                       ` Junxiao Bi
2020-06-23  0:47                     ` Matthew Wilcox
2020-06-25 22:11                       ` Junxiao Bi
2020-06-22  5:33         ` Masahiro Yamada [this message]
2020-06-22 15:13           ` Eric W. Biederman

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