linux-kernel - Re: [PATCH v7 1/4] spinlock: A new lockref structure for lockless update of refcount

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Date:	Mon, 02 Sep 2013 15:25:59 -0400
From:	Waiman Long <waiman.long@...com>
To:	Al Viro <viro@...IV.linux.org.uk>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Jeff Layton <jlayton@...hat.com>,
	Miklos Szeredi <mszeredi@...e.cz>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Andi Kleen <andi@...stfloor.org>,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	"Norton, Scott J" <scott.norton@...com>
Subject: Re: [PATCH v7 1/4] spinlock: A new lockref structure for lockless
 update of refcount

On 08/30/2013 10:42 PM, Al Viro wrote:
> On Sat, Aug 31, 2013 at 03:35:16AM +0100, Al Viro wrote:
>
>> Aha...  OK, I see what's going on.  We end up with shm_mnt *not* marked
>> as long-living vfsmount, even though it lives forever.  See if the
>> following helps; if it does (and I very much expect it to), we want to
>> put it in -stable.  As it is, you get slow path in mntput() each time
>> a file created by shmem_file_setup() gets closed.  For no reason whatsoever...
> We still want MS_NOUSER on shm_mnt, so we'd better make sure that
> shmem_fill_super() sets it on the internal instance...  Fixed variant
> follows:
>
> Signed-off-by: Al Viro<viro@...iv.linux.org.uk>
> diff --git a/mm/shmem.c b/mm/shmem.c
> index e43dc55..5261498 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -2615,13 +2615,15 @@ int shmem_fill_super(struct super_block *sb, void *data, int silent)
>   	 * tmpfs instance, limiting inodes to one per page of lowmem;
>   	 * but the internal instance is left unlimited.
>   	 */
> -	if (!(sb->s_flags&  MS_NOUSER)) {
> +	if (!(sb->s_flags&  MS_KERNMOUNT)) {
>   		sbinfo->max_blocks = shmem_default_max_blocks();
>   		sbinfo->max_inodes = shmem_default_max_inodes();
>   		if (shmem_parse_options(data, sbinfo, false)) {
>   			err = -EINVAL;
>   			goto failed;
>   		}
> +	} else {
> +		sb->s_flags |= MS_NOUSER;
>   	}
>   	sb->s_export_op =&shmem_export_ops;
>   	sb->s_flags |= MS_NOSEC;
> @@ -2831,8 +2833,7 @@ int __init shmem_init(void)
>   		goto out2;
>   	}
>
> -	shm_mnt = vfs_kern_mount(&shmem_fs_type, MS_NOUSER,
> -				 shmem_fs_type.name, NULL);
> +	shm_mnt = kern_mount(&shmem_fs_type);
>   	if (IS_ERR(shm_mnt)) {
>   		error = PTR_ERR(shm_mnt);
>   		printk(KERN_ERR "Could not kern_mount tmpfs\n");

Yes, that patch worked. It eliminated the lglock as a bottleneck in the 
AIM7 workload. The lg_global_lock did not show up in the perf profile, 
whereas the lg_local_lock was only 0.07%.

Regards,
Longman
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