linux-kernel - Re: [PATCH 2/5] mm: remove unlikly NULL from kfree

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Message-ID: <alpine.DEB.2.00.0903251103070.5675@gandalf.stny.rr.com>
Date:	Wed, 25 Mar 2009 11:04:06 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Pekka Enberg <penberg@...helsinki.fi>
cc:	Hua Zhong <hzhong@...il.com>, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Roland McGrath <roland@...hat.com>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Steven Rostedt <srostedt@...hat.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Matt Mackall <mpm@...enic.com>,
	Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH 2/5] mm: remove unlikly NULL from kfree


On Wed, 25 Mar 2009, Pekka Enberg wrote:

> Hi Steven,
> 
> On Wed, 25 Mar 2009, Pekka Enberg wrote:
> > > OK, so according to Steven, audit_syscall_exit() is one such call-site
> > > that shows up in the traces. I don't really understand what's going on
> > > there but if it is sane, maybe that warrants the removal of unlikely()
> > > from kfree(). Hmm?
> 
> On Wed, 2009-03-25 at 10:47 -0400, Steven Rostedt wrote:
> > After disabling AUDIT_SYSCALLS I have this:
> > 
> >  # cat /debug/tracing/trace | sort -u
> > 
> > record_nulls: ptr=(null) (ext3_get_acl+0x1e0/0x3f0 [ext3])
> > record_nulls: ptr=(null) (free_bitmap+0x29/0x70)
> > record_nulls: ptr=(null) (free_tty_struct+0x1d/0x40)
> > record_nulls: ptr=(null) (ftrace_graph_exit_task+0x1e/0x20)
> > record_nulls: ptr=(null) (inet_sock_destruct+0x1cb/0x2a0)
> > record_nulls: ptr=(null) (ip_cork_release+0x24/0x50)
> > record_nulls: ptr=(null) (keyctl_join_session_keyring+0x5a/0x70)
> > record_nulls: ptr=(null) (key_user_lookup+0x183/0x220)
> > record_nulls: ptr=(null) (kobject_set_name_vargs+0x43/0x50)
> > record_nulls: ptr=(null) (netlink_release+0x1a4/0x2f0)
> > record_nulls: ptr=(null) (release_sysfs_dirent+0x20/0xc0)
> > record_nulls: ptr=(null) (sysfs_open_file+0x1c8/0x3e0)
> > record_nulls: ptr=(null) (tty_write+0x16a/0x290)
> > 
> > I added a hook to only record when NULL is passed into kfree.
> > 
> > Also note, that after disabling AUDIT_SYSCALLS I now only have roughly 7% 
> > NULL hit rate. Still, unlikely is probably not a benefit here.
> 
> Thanks for doing this. Do you mean that 93% hit ratio is not enough to
> be a performance gain?

I think it was Christoph Lameter (good you Cc'd him) told me that anything 
less that 99% is not worthy of a (un)likely macro.

I honestly don't know.

-- Steve

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