From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756646AbaEIXrv (ORCPT ); Fri, 9 May 2014 19:47:51 -0400 Received: from e38.co.us.ibm.com ([32.97.110.159]:37249 "EHLO e38.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754078AbaEIXru (ORCPT ); Fri, 9 May 2014 19:47:50 -0400 Date: Fri, 9 May 2014 16:47:45 -0700 From: "Paul E. McKenney" To: Thomas Gleixner Cc: Christoph Lameter , Andrew Morton , Gilad Ben-Yossef , Tejun Heo , Mike Frysinger , Minchan Kim , Hakan Akkan , Max Krasnyansky , Frederic Weisbecker , LKML , linux-mm@kvack.org, hughd@google.com, viresh.kumar@linaro.org, Ingo Molnar , "H. Peter Anvin" , Peter Zijlstra , John Stultz Subject: Re: vmstat: On demand vmstat workers V4 Message-ID: <20140509234745.GB8754@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20140508142903.c2ef166c95d2b8acd0d7ea7d@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 14050923-1344-0000-0000-000001757D52 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, May 10, 2014 at 12:57:15AM +0200, Thomas Gleixner wrote: > On Fri, 9 May 2014, Christoph Lameter wrote: > > On Fri, 9 May 2014, Thomas Gleixner wrote: > > > I understand why you want to get this done by a housekeeper, I just > > > did not understand why we need this whole move it around business is > > > required. > > > > This came about because of another objection against having it simply > > fixed to a processor. After all that processor may be disabled etc etc. > > I really regret that I did not pay more attention (though my cycle > constraints simply do not allow it). As far as I can see, the NO_HZ_FULL timekeeping CPU is always zero. If it can change in NO_HZ_FULL kernels, RCU will do some very strange things! One possible issue here is that Christoph's patch is unconditional. It takes effect for both NO_HZ_FULL and !NO_HZ_FULL. If I recall correctly, the timekeeping CPU -can- change in !NO_HZ_FULL kernels, which might be what Christoph was trying to take into account. > This is the typical overengineering failure: > > Before we even have a working proof that we can solve the massive > complex basic problem with the price of a dedicated housekeeper, we > try to make the housekeeper itself a moving target with the price of > making the problem exponential(unknown) instead of simply unknown. > > I really cannot figure out why a moving housekeeper would be a > brilliant idea at all, but I'm sure there is some magic use case in > some other disjunct universe. > > Whoever complained and came up with the NOT SO brilliant idea to make > the housekeeper a moving target, come please forth and explain: > > - How this can be done without having a working solution with a > dedicated housekeeper in the first place > > - How this can be done without knowing what implication it has w/o > seing the complexity of a dedicated housekeeper upfront. > > Keep it simple has always been and still is the best engineering > principle. If someone decides to make tick_do_timer_cpu non-constant in NO_HZ_FULL CPUs, they will break unless/until I make RCU deal with that sort of thing, at least for NO_HZ_FULL_SYSIDLE kernels. ;-) > We all know that we can do large scale overhauls in a very controlled > way if the need arises. But going for the most complex solution while > not knowing whether the least complex solution is feasible at all is > outright stupid or beyond. > > Unless someone comes up with a reasonable explantion for all of this I > put a general NAK on patches which are directed to kernel/time/* > > Correction: > > I'm taking patches right away which undo any damage which has been > applied w/o me noticing because I trusted the responsible developers / > maintainers. > > Preferrably those patches arrive before my return from LinuxCon Japan. I could easily have missed something, but as far as I know, there is nothing in the current kernel that allows tick_do_timer_cpu to move in NO_HZ_FULL kernels. Hmmm... Well, I -do- have a gratuitous ACCESS_ONCE() around one fetch of tick_do_timer_cpu that happens only in NO_HZ_FULL kernels. I will remove it. Thanx, Paul > Thanks, > > tglx >