From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754033Ab0A0KBl (ORCPT ); Wed, 27 Jan 2010 05:01:41 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753735Ab0A0KBl (ORCPT ); Wed, 27 Jan 2010 05:01:41 -0500 Received: from e3.ny.us.ibm.com ([32.97.182.143]:59616 "EHLO e3.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751051Ab0A0KBk (ORCPT ); Wed, 27 Jan 2010 05:01:40 -0500 Date: Wed, 27 Jan 2010 02:01:36 -0800 From: "Paul E. McKenney" To: Andi Kleen 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, josh@joshtriplett.org, dvhltc@us.ibm.com, niv@us.ibm.com, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, Valdis.Kletnieks@vt.edu, dhowells@redhat.com Subject: Re: [PATCH RFC tip/core/rcu] accelerate grace period if last non-dynticked CPU Message-ID: <20100127100136.GM6807@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20100125034816.GA14043@linux.vnet.ibm.com> <873a1sft9q.fsf@basil.nowhere.org> <20100127052050.GC6807@linux.vnet.ibm.com> <20100127094336.GA12522@basil.fritz.box> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20100127094336.GA12522@basil.fritz.box> User-Agent: Mutt/1.5.15+20070412 (2007-04-11) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jan 27, 2010 at 10:43:36AM +0100, Andi Kleen wrote: > On Tue, Jan 26, 2010 at 09:20:50PM -0800, Paul E. McKenney wrote: > > On Tue, Jan 26, 2010 at 10:30:57PM +0100, Andi Kleen wrote: > > > "Paul E. McKenney" writes: > > > > > > Kind of offtopic to the original patch, but I couldn't > > > resist... > > > > > > > +config RCU_FAST_NO_HZ > > > > + bool "Accelerate last non-dyntick-idle CPU's grace periods" > > > > + depends on TREE_RCU && NO_HZ && SMP > > > > > > Having such a thing as a config option doesn't really make > > > any sense to me. Who would want to recompile their kernel > > > to enable/disable this? If anything it should be runtime, or better > > > just unconditionally on. > > > > It adds significant overhead on entry to dyntick-idle mode for systems > > with large numbers of CPUs. :-( > > Can't you simply check that at runtime then? > > if (num_possible_cpus() > 20) > ... > > BTW the new small is large. This years high end desktop PC will come with > upto 12 CPU threads. It would likely be challenging to find a good > number for 20 that holds up with the future. And this was another line of reasoning that lead me to the extra kernel config parameter. > Or better perhaps have some threshold that you don't do it > that often, or only do it when you expect to be idle for a long > enough time that the CPU can enter deeper idle states > > (I higher idle states some more wakeups typically don't matter > that much) > > The cpufreq/cstate governour have a reasonable good idea > now how "idle" the system is and will be. Maybe you can reuse > that information somehow. My first thought was to find an existing "I am a small device running on battery power" or "low power consumption is critical to me" config parameter. I didn't find anything that looked like that. If there was one, I would make RCU_FAST_NO_HZ depend on it. Or did I miss some kernel parameter or API? Thanx, Paul