linux-kernel - Re: RFC: THE OFFLINE SCHEDULER

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Message-ID: <20090826213400.GA25536@elte.hu>
Date:	Wed, 26 Aug 2009 23:34:00 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Christoph Lameter <cl@...ux-foundation.org>, peterz@...radead.org,
	raziebe@...il.com, maximlevitsky@...il.com, cfriesen@...tel.com,
	efault@....de, riel@...hat.com, wiseman@...s.biu.ac.il,
	linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org
Subject: Re: RFC: THE OFFLINE SCHEDULER

* Andrew Morton <akpm@...ux-foundation.org> wrote:

> On Wed, 26 Aug 2009 16:40:09 -0400 (EDT)
> Christoph Lameter <cl@...ux-foundation.org> 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
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