From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753150Ab3HSBjg (ORCPT ); Sun, 18 Aug 2013 21:39:36 -0400 Received: from e34.co.us.ibm.com ([32.97.110.152]:35731 "EHLO e34.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752796Ab3HSBje (ORCPT ); Sun, 18 Aug 2013 21:39:34 -0400 Date: Sun, 18 Aug 2013 18:39:25 -0700 From: "Paul E. McKenney" To: Josh Triplett Cc: linux-kernel@vger.kernel.org, mingo@elte.hu, laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@polymtl.ca, niv@us.ibm.com, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, darren@dvhart.com, fweisbec@gmail.com, sbw@mit.edu Subject: Re: [PATCH tip/core/rcu 6/9] nohz_full: Add full-system idle states and variables Message-ID: <20130819013924.GA29406@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20130818014918.GA27827@linux.vnet.ibm.com> <1376790584-28120-1-git-send-email-paulmck@linux.vnet.ibm.com> <1376790584-28120-6-git-send-email-paulmck@linux.vnet.ibm.com> <20130818030920.GI28923@leaf> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20130818030920.GI28923@leaf> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: No X-Content-Scanned: Fidelis XPS MAILER x-cbid: 13081901-1542-0000-0000-0000009FA985 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, Aug 17, 2013 at 08:09:21PM -0700, Josh Triplett wrote: > On Sat, Aug 17, 2013 at 06:49:41PM -0700, Paul E. McKenney wrote: > > From: "Paul E. McKenney" > > > > This commit adds control variables and states for full-system idle. > > The system will progress through the states in numerical order when > > the system is fully idle (other than the timekeeping CPU), and reset > > down to the initial state if any non-timekeeping CPU goes non-idle. > > The current state is kept in full_sysidle_state. > > > > A RCU_SYSIDLE_SMALL macro is defined, and systems with this number > > of CPUs or fewer move through the states more aggressively. The idea > > is that the resulting memory contention is less of a problem on small > > systems. Architectures can adjust this value (which defaults to 8) > > using CONFIG_ARCH_RCU_SYSIDLE_SMALL. > > > > One flavor of RCU will be in charge of driving the state machine, > > defined by rcu_sysidle_state. This should be the busiest flavor of RCU. > > > > Signed-off-by: Paul E. McKenney > > Cc: Frederic Weisbecker > > Cc: Steven Rostedt > > One issue (and one question) below; with the issue addressed, > Reviewed-by: Josh Triplett > > > kernel/rcutree_plugin.h | 28 ++++++++++++++++++++++++++++ > > 1 file changed, 28 insertions(+) > > > > diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h > > index eab81da..64a05b9f 100644 > > --- a/kernel/rcutree_plugin.h > > +++ b/kernel/rcutree_plugin.h > > @@ -2378,6 +2378,34 @@ static void rcu_kick_nohz_cpu(int cpu) > > #ifdef CONFIG_NO_HZ_FULL_SYSIDLE > > > > /* > > + * Handle small systems specially, accelerating their transition into > > + * full idle state. Allow arches to override this code's idea of > > + * what constitutes a "small" system. > > + */ > > +#ifdef CONFIG_ARCH_RCU_SYSIDLE_SMALL > > I don't see any Kconfig creating this new config option. > > Also, why not simply define this config option unconditionally, with a > default of 8, and then use its value directly? Good point, removing this and adding a Kconfig option in the "nohz_full: Add full-system-idle state machine" commit, with a default value of 8. Architecture maintainers who want something different can then set that up in their defconfig files. > > +static int __maybe_unused full_sysidle_state; /* Current system-idle state. */ > > +#define RCU_SYSIDLE_NOT 0 /* Some CPU is not idle. */ > > +#define RCU_SYSIDLE_SHORT 1 /* All CPUs idle for brief period. */ > > +#define RCU_SYSIDLE_LONG 2 /* All CPUs idle for long enough. */ > > +#define RCU_SYSIDLE_FULL 3 /* All CPUs idle, ready for sysidle. */ > > +#define RCU_SYSIDLE_FULL_NOTED 4 /* Actually entered sysidle state. */ > > Perhaps there's a kernel style rule I'm not thinking of that makes it > verboten, but: why not use an enum for a state variable like this? I didn't trust enum interactions with xchg and cmpxchg, so opted for "int" instead. That said, enum is much more portable than when I last looked at it. Admittedly, the last time I looked at it was in the early 1980s... Thanx, Paul