linux-kernel - Re: [PATCH] sched: properly account IRQ and RT load in SCHED

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Date:	Thu, 21 Aug 2008 14:56:42 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Gregory Haskins <ghaskins@...ell.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Nick Piggin <nickpiggin@...oo.com.au>,
	vatsa <vatsa@...ibm.com>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched: properly account IRQ and RT load in SCHED_OTHER
	load balancing

On Thu, 2008-08-21 at 08:47 -0400, Gregory Haskins wrote:
> Peter Zijlstra wrote:
> > OK, how overboard is this? (utterly uncompiled and such)
> >
> > I realized while trying to do the (soft)irq accounting Ingo asked for,
> > that IRQs can preempt SoftIRQs which can preempt RT tasks.
> >
> > Therefore we actually need to account all these times, so that we can
> > subtract irq time from measured softirq time, etc.
> >
> > So this patch does all that.. we could even use this more accurate time
> > spend on the task delta to drive the scheduler.
> >
> > NOTE - for now I've only considered softirq from hardirq time, as
> > ksoftirqd is its own task and is already accounted the regular way.
> >   
> 
> Actually, if you really want to get crazy, you could account for each RT 
> prio level as well ;)
> 
> e.g. RT98 tasks have to account for RT99 + softirqs + irqs, RT97 need to 
> look at RT98, 99, softirqs, irqs, etc.
> 
> I'm not suggesting we do this, per se.   Just food for thought.  It 
> would have the benefit of allowing us to make even better routing 
> decisions for RT tasks.  E.g. if cores 2 and 6 both have the lowest 
> priority, we currently sort by sched-domain topology, but we could also 
> factor in the load that is "above" us.

I'll let you be that crazy ;-) It'd be a 3-rd order placement decision,
I doubt that's going to make a large difference.

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