Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755855Ab3DLXyT (ORCPT ); Fri, 12 Apr 2013 19:54:19 -0400 Received: from relay5-d.mail.gandi.net ([217.70.183.197]:36431 "EHLO relay5-d.mail.gandi.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753538Ab3DLXyR (ORCPT ); Fri, 12 Apr 2013 19:54:17 -0400 X-Originating-IP: 173.246.103.110 Date: Fri, 12 Apr 2013 16:54:02 -0700 From: Josh Triplett To: "Paul E. McKenney" 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, Valdis.Kletnieks@vt.edu, dhowells@redhat.com, edumazet@google.com, darren@dvhart.com, fweisbec@gmail.com, sbw@mit.edu Subject: Re: [PATCH tip/core/rcu 6/7] rcu: Drive quiescent-state-forcing delay from HZ Message-ID: <20130412235401.GA8140@jtriplet-mobl1> References: <20130412231846.GA20038@linux.vnet.ibm.com> <1365808754-20762-1-git-send-email-paulmck@linux.vnet.ibm.com> <1365808754-20762-6-git-send-email-paulmck@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1365808754-20762-6-git-send-email-paulmck@linux.vnet.ibm.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1895 Lines: 46 On Fri, Apr 12, 2013 at 04:19:13PM -0700, Paul E. McKenney wrote: > From: "Paul E. McKenney" > > Systems with HZ=100 can have slow bootup times due to the default > three-jiffy delays between quiescent-state forcing attempts. This > commit therefore auto-tunes the RCU_JIFFIES_TILL_FORCE_QS value based > on the value of HZ. However, this would break very large systems that > require more time between quiescent-state forcing attempts. This > commit therefore also ups the default delay by one jiffy for each > 256 CPUs that might be on the system (based off of nr_cpu_ids at > runtime, -not- NR_CPUS at build time). > > Reported-by: Paul Mackerras > Signed-off-by: Paul E. McKenney Something seems very wrong if RCU regularly hits the fqs code during boot; feels like there's some more straightforward solution we're missing. What causes these CPUs to fall under RCU's scrutiny during boot yet not actually hit the RCU codepaths naturally? Also, a comment below. > --- a/kernel/rcutree.h > +++ b/kernel/rcutree.h > @@ -342,7 +342,17 @@ struct rcu_data { > #define RCU_FORCE_QS 3 /* Need to force quiescent state. */ > #define RCU_SIGNAL_INIT RCU_SAVE_DYNTICK > > -#define RCU_JIFFIES_TILL_FORCE_QS 3 /* for rsp->jiffies_force_qs */ > +#if HZ > 500 > +#define RCU_JIFFIES_TILL_FORCE_QS 3 /* for jiffies_till_first_fqs */ > +#elif HZ > 250 > +#define RCU_JIFFIES_TILL_FORCE_QS 2 > +#else > +#define RCU_JIFFIES_TILL_FORCE_QS 1 > +#endif This seems like it really wants to use a duration calculated directly from HZ; perhaps (HZ/100)? - Josh Triplett -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/