linux-kernel - Re: AIM7 40% regression with 2.6.26-rc1

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Message-ID: <20080507062639.GA26031@elte.hu>
Date:	Wed, 7 May 2008 08:26:39 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
Cc:	Matthew Wilcox <matthew@....cx>,
	LKML <linux-kernel@...r.kernel.org>,
	Alexander Viro <viro@....linux.org.uk>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: AIM7 40% regression with 2.6.26-rc1


* Zhang, Yanmin <yanmin_zhang@...ux.intel.com> wrote:

> 3) Caller of lock_kernel are sys_fcntl/vfs_ioctl/tty_release/chrdev_open.

that's one often-forgotten BKL site: about 1000 ioctls are still running 
under the BKL. The TTY one is hurting the most. To make sure it's only 
that BKL acquire/release that hurts, could you try the hack patch below, 
does it make any difference to performance?

but even if taking the BKL does hurt, it's quite unexpected to cause a 
40% drop. Perhaps AIM7 has tons of threads that exit at once and all try 
to release their controlling terminal or something like that?

	Ingo

------------------------>
Subject: DANGEROUS tty hack: no BKL
From: Ingo Molnar <mingo@...e.hu>
Date: Wed May 07 08:21:22 CEST 2008

NOT-Signed-off-by: Ingo Molnar <mingo@...e.hu>
---
 drivers/char/tty_io.c |    5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

Index: linux/drivers/char/tty_io.c
===================================================================
--- linux.orig/drivers/char/tty_io.c
+++ linux/drivers/char/tty_io.c
@@ -2844,9 +2844,10 @@ out:
 
 static int tty_release(struct inode *inode, struct file *filp)
 {
-	lock_kernel();
+	/* DANGEROUS - can crash your kernel! */
+//	lock_kernel();
 	release_dev(filp);
-	unlock_kernel();
+//	unlock_kernel();
 	return 0;
 }
 
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