linux-kernel - Re: [PATCH 0/6] x86/cpu hotplug: Wake up offline CPU via mwait or nmi

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Date:	Tue, 5 Jun 2012 23:15:16 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Andi Kleen <andi@...stfloor.org>
cc:	Peter Zijlstra <peterz@...radead.org>,
	"Luck\\, Tony" <tony.luck@...el.com>,
	"Yu\\, Fenghua" <fenghua.yu@...el.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Ingo Molnar <mingo@...e.hu>, H Peter Anvin <hpa@...or.com>,
	"Siddha\\, Suresh B" <suresh.b.siddha@...el.com>,
	"Mallick\\, Asit K" <asit.k.mallick@...el.com>,
	Arjan Dan De Ven <arjan@...ux.intel.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	x86 <x86@...nel.org>, linux-pm <linux-pm@...r.kernel.org>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Subject: Re: [PATCH 0/6] x86/cpu hotplug: Wake up offline CPU via mwait or
 nmi

On Tue, 5 Jun 2012, Andi Kleen wrote:
> Thomas Gleixner <tglx@...utronix.de> writes:
> >
> > Vs. the interrupt/timer/other crap madness:
> >
> >  - We really don't want to have an interrupt balancer in the kernel
> >    again, but we need a mechanism to prevent the user space balancer
> >    trainwreck from ruining the power saving party.
> 
> Why not? I think the kernel is exactly the right place for it.
> It's essentially a scheduling problem. Scheduling in user space
> is not a good idea.

No argument about scheduling in user space. Though the real problem is
where do you draw the line between mechanism and policy?
 
> With MSI-X the drivers just want a static setting. User space
> shouldn't mess with it. 
> 
> Some of the workarounds for user space messing with it (like that
> interrupt rmap code) are really bad and just a workaround for doing the
> scheduling in the wrong place.
> 
> For dynamic changes it should indeed by part of scheduling,
> following similar rules, with only high level policy input
> from userland.

I'd be happy to see a patch which implements all of that and avoids
the pitfalls of the old in kernel irq balancer along with the short
comings of the user space one.

Thanks,

	tglx
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