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

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Date:	Tue, 3 Sep 2013 08:01:30 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Waiman Long <waiman.long@...com>
Cc:	Al Viro <viro@...IV.linux.org.uk>,
	Linus Torvalds <torvalds@...ux-foundation.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


* Waiman Long <waiman.long@...com> wrote:

> 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%.

Just curious: what's the worst bottleneck now in the optimized kernel? :-)

Thanks,

	Ingo
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