From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ingo Molnar Subject: Re: [GIT RFC PULL rcu/urgent] Prevent Kconfig from asking pointless questions Date: Mon, 20 Apr 2015 20:28:32 +0200 Message-ID: <20150420182831.GA19510@gmail.com> References: <20150416183812.GA5571@linux.vnet.ibm.com> <20150418130340.GA26931@gmail.com> <20150418133444.GD23685@linux.vnet.ibm.com> <20150418143238.GA2337@gmail.com> <20150419020541.GA5561@linux.vnet.ibm.com> <20150420113554.598e503f@sluggy> <20150420170902.GU5561@linux.vnet.ibm.com> <20150420180107.GE24936@home.goodmis.org> <20150420180904.GA19229@gmail.com> <20150420142149.3ac58a2c@gandalf.local.home> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: "Paul E. McKenney" , Clark Williams , linux-kernel@vger.kernel.org, torvalds@linux-foundation.org, a.p.zijlstra@chello.nl, akpm@linux-foundation.org, linux-rt-users@vger.kernel.org To: Steven Rostedt Return-path: Content-Disposition: inline In-Reply-To: <20150420142149.3ac58a2c@gandalf.local.home> Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-rt-users.vger.kernel.org * Steven Rostedt wrote: > On Mon, 20 Apr 2015 20:09:04 +0200 > Ingo Molnar wrote: > > > > So the disadvantage is that if a boot default is wrong, we'll hear > > about it eventually and can fix/improve it. > > > > If a sysctl knob is wrong, people will just 'tune' it and forget > > to propagate it to the kernel proper (why should they). > > My fear is that there is no one true value. [...] Do we know that? > [...] One person complains about it, we change it, then someone else > complains about the new value. That would be even worse. At that point we can still add a sysctl, if valid arguments are offered. > > Which is fine for something like ftrace and other ad-hoc > > instrumentation that is generally very fine tuned to a given bug > > or given piece of hardware, but for something like the RCU > > implementation of the kernel - even if it's just a RT side thought > > of it - I'm not so sure about it. > > I would argue than every case is different, and only the sysadmin > would know the right value. Thus, just set it to one, and if that's > not good enough, then the sysadmins can change it to their needs. Well, we had really bad experience with sysctls in the past, in particular in the VM: with various settings exposed and distros 'tuning' them - sometimes radically changing the way the system worked, confusing everyone involved. So I'm in general opposed to sysctls for core kernel behavior - except for cases where we don't know better. Instrumentation - especially instrumentation that should have been implemented mostly in user-space, like ftrace ;-) - is another special case that should stay as flexible as possible via sysctls, obviously. Thanks, Ingo