linux-kernel - [RFC] Move kfree outside pde_unload

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Message-ID: <20120821205454.GA7772@gulag1.americas.sgi.com>
Date:	Tue, 21 Aug 2012 15:54:54 -0500
From:	Nathan Zimmer <nzimmer@....com>
To:	Alexander Viro <viro@...iv.linux.org.uk>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	adobriyan@...il.com
Subject: [RFC] Move kfree outside pde_unload_lock

I am currently tracking a hotlock reported by a customer on a large, 512 cores,
system, I am currently running 3.6.0 rc1 but the issue looks like it has been
this way for a very long time.
The offending lock is proc_dir_entry->pde_unload_lock.  

In proc_reg_release we are doing a kfree under the spinlock which is ok but it
means we are holding the lock longer then required. Scaling improved when I 
moved kfree out.

Also shouldn't the comment on pde_unload_lock also note that pde_openers and 
pde_unload_completion are both used under the lock?

Here is some data from quick test program which just reads from /proc/cpuinfo.
Lower is better, as you can see the worst case scenario is improved.
	baseline	moved kfree	
tasks	read-sec	read-sec	
1	0.0141		0.0141
2	0.0140		0.0140
4	0.0140		0.0141
8	0.0145		0.0145
16	0.0553		0.0548
32	0.1688		0.1622
64	0.5017		0.3856
128	1.7005		0.9710
256	5.2513		2.6519
512	8.0529		6.2976

If the patch looks agreeable I will resend it properly.

diff --git a/fs/proc/inode.c b/fs/proc/inode.c
index 7ac817b..46016c1 100644
--- a/fs/proc/inode.c
+++ b/fs/proc/inode.c
@@ -403,9 +403,11 @@ static int proc_reg_release(struct inode *inode, struct file *file)
 	release = pde->proc_fops->release;
 	if (pdeo) {
 		list_del(&pdeo->lh);
-		kfree(pdeo);
 	}
 	spin_unlock(&pde->pde_unload_lock);
+	if (pdeo) {
+		kfree(pdeo);
+	}

 	if (release)
 		rv = release(inode, file);

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