From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755251AbZHMVXr (ORCPT ); Thu, 13 Aug 2009 17:23:47 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752297AbZHMVXq (ORCPT ); Thu, 13 Aug 2009 17:23:46 -0400 Received: from smtp1.linux-foundation.org ([140.211.169.13]:40966 "EHLO smtp1.linux-foundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751690AbZHMVXq (ORCPT ); Thu, 13 Aug 2009 17:23:46 -0400 Date: Thu, 13 Aug 2009 14:21:44 -0700 From: Andrew Morton To: Roland Dreier Cc: linux-kernel@vger.kernel.org, x86@kernel.org Subject: Re: [PATCH] Use bool for boolean flag in printk_once() Message-Id: <20090813142144.37509d52.akpm@linux-foundation.org> In-Reply-To: References: X-Mailer: Sylpheed version 2.2.4 (GTK+ 2.8.20; i486-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 13 Aug 2009 13:48:26 -0700 Roland Dreier wrote: > Using the type bool (instead of int) for the __print_once flag in the > printk_once() macro matches the intent of the code better, and allows > the compiler to generate smaller code; eg a typical callsite with gcc > 4.3.3 on i386: > > add/remove: 0/0 grow/shrink: 0/2 up/down: 0/-6 (-6) > function old new delta > static.__print_once 4 1 -3 > get_cpu_vendor 146 143 -3 > > Saving 6 bytes of object size per callsite by slightly improving the > readability of the source seems like a win to me. > > Signed-off-by: Roland Dreier > --- > include/linux/kernel.h | 4 ++-- > 1 files changed, 2 insertions(+), 2 deletions(-) > > diff --git a/include/linux/kernel.h b/include/linux/kernel.h > index d6320a3..f828ce9 100644 > --- a/include/linux/kernel.h > +++ b/include/linux/kernel.h > @@ -249,10 +249,10 @@ extern bool printk_timed_ratelimit(unsigned long *caller_jiffies, > * Print a one-time message (analogous to WARN_ONCE() et al): > */ > #define printk_once(x...) ({ \ > - static int __print_once = 1; \ > + static bool __print_once = true; \ > \ > if (__print_once) { \ > - __print_once = 0; \ > + __print_once = false; \ > printk(x); \ > } \ > }) hm, OK, in trace_recursive_lock() we get: cmpl $0, __print_once.28104(%rip) #, __print_once je .L719 #, changed to cmpb $0, __print_once.28104(%rip) # __print_once je .L719 #, so the compiler used a byte for the bool. Interestingly that bool is in section `data'. I bet printk_once() should use __read_mostly. Also, that bool still uses four bytes of storage: text data bss dec hex filename 22527 1445 6392 30364 769c kernel/trace/ring_buffer.o 22524 1445 6392 30361 7699 kernel/trace/ring_buffer.o (patched) I wonder if there's some way in which we could/should cause all these little random __read_mostly bytes to get packed together somewhere.