linux-kernel - Re: [PATCH] rcu: Deinline rcu_read_lock_sched_held() if DEBUG_LOCK

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Date:	Thu, 21 May 2015 09:09:43 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Denys Vlasenko <dvlasenk@...hat.com>,
	Josh Triplett <josh@...htriplett.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	linux-kernel@...r.kernel.org, Tejun Heo <tj@...nel.org>,
	Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH] rcu: Deinline rcu_read_lock_sched_held() if
 DEBUG_LOCK_ALLOC

On Thu, 21 May 2015 05:52:24 -0700
"Paul E. McKenney" <paulmck@...ux.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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