From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ingo Molnar Subject: Re: RFC: THE OFFLINE SCHEDULER Date: Wed, 26 Aug 2009 23:34:00 +0200 Message-ID: <20090826213400.GA25536@elte.hu> References: <1251298443.4791.7.camel@raz> <1251300625.18584.18.camel@twins> <1251302598.18584.31.camel@twins> <20090826180407.GA13632@elte.hu> <20090826193252.GA14721@elte.hu> <20090826135041.e6169d18.akpm@linux-foundation.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Christoph Lameter , peterz@infradead.org, raziebe@gmail.com, maximlevitsky@gmail.com, cfriesen@nortel.com, efault@gmx.de, riel@redhat.com, wiseman@macs.biu.ac.il, linux-kernel@vger.kernel.org, linux-rt-users@vger.kernel.org To: Andrew Morton Return-path: Content-Disposition: inline In-Reply-To: <20090826135041.e6169d18.akpm@linux-foundation.org> Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-rt-users.vger.kernel.org * Andrew Morton wrote: > On Wed, 26 Aug 2009 16:40:09 -0400 (EDT) > Christoph Lameter wrote: > > > Peter has not given a solution to the problem. Nor have you. > > What problem? > > All I've seen is "I want 100% access to a CPU". That's not a problem > statement - it's an implementation. > > What is the problem statement? > > My take on these patches: the kernel gives userspace unmediated > access to memory resources if it wants that. The kernel gives > userspace unmediated access to IO devices if it wants that. But > for some reason people freak out at the thought of providing > unmediated access to CPU resources. Claiming all user-available CPU time from user-space is already possible: use SCHED_FIFO - the only question are remaining latencies in the final 0.01% of CPU time you cannot claim via SCHED_FIFO. ( Btw., this scheduling feature was implemented in Linux well before raw IO block devices were implemented, so i'm not sure what you mean by 'freaking out'. ) What we are objecting to are these easy isolation side-hacks for the remaining 0.01% that fail to address the real problem: the latencies. Those latencies can hurt not just isolated apps but _non isolated_ (and latency critical) apps too, and what we insist on is getting the proper fixes, not just ugly workarounds that side-step the problem. ( a secondary objection is the extension and extra layering of something that could be done within existing APIs/ABIs too. We want to minimize the configuration space. ) > Don't get all religious about this. If the change is clean, > maintainable and useful then there's no reason to not merge it. Precisely. This feature as proposed here hinders the correct solution being implemented - and hence hurts long term maintainability and hence is a no-merge right now. [It also weakens the pressure to fix latencies for a much wider set of applications, hence hurts the quality of Linux in the long run. (i.e. is a net step backwards)] Ingo