Date: Thu, 21 May 2015 09:09:43 -0400
From: Steven Rostedt <rostedt@goodmis.org>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>,
        Josh Triplett <josh@joshtriplett.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Lai Jiangshan <laijs@cn.fujitsu.com>, linux-kernel@vger.kernel.org,
        Tejun Heo <tj@kernel.org>, Oleg Nesterov <oleg@redhat.com>
Subject: Re: [PATCH] rcu: Deinline rcu_read_lock_sched_held() if
 DEBUG_LOCK_ALLOC
Message-ID: <20150521090943.0e01ed05@gandalf.local.home>
In-Reply-To: <20150521125224.GK6776@linux.vnet.ibm.com>
References: <1432202647-20201-1-git-send-email-dvlasenk@redhat.com>
	<20150521125224.GK6776@linux.vnet.ibm.com>
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On Thu, 21 May 2015 05:52:24 -0700
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:

> On Thu, May 21, 2015 at 12:04:07PM +0200, Denys Vlasenko wrote:
> > DEBUG_LOCK_ALLOC=y is not a production setting, but it is
> > not very unusual either. Many developers routinely
> > use kernels built with it enabled.
> > 
> > Apart from being selected by hand, it is also auto-selected by
> > PROVE_LOCKING "Lock debugging: prove locking correctness" and
> > LOCK_STAT "Lock usage statistics" config options.
> > LOCK STAT is necessary for "perf lock" to work.
> > 
> > I wouldn't spend too much time optimizing it, but this particular
> > function has a very large cost in code size: when it is deinlined,
> > code size decreases by 830,000 bytes:
> > 
> >     text     data      bss       dec     hex filename
> > 85674192 22294776 20627456 128596424 7aa39c8 vmlinux.before
> > 84837612 22294424 20627456 127759492 79d7484 vmlinux
> > 
> > (with this config: http://busybox.net/~vda/kernel_config)
> 
> OK, I'll bite...  I do see the numbers above, but is this really a
> problem for anyone?  As you say, DEBUG_LOCK_ALLOC=y is not a production
> setting.
> 

Correct, and because it's not a production setting it should be fine as
a call and not a static inline. The i$ hit probably neglects the saving
of it being inlined too.

It's not a big deal either way, but it may make building the kernel a
bit faster ;-)

-- Steve
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