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

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Message-ID: <20120605212947.GA8686@linux.vnet.ibm.com>
Date:	Tue, 5 Jun 2012 14:29:47 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	"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, Jun 05, 2012 at 09:49:16PM +0200, Peter Zijlstra wrote:
> On Tue, 2012-06-05 at 21:43 +0200, Thomas Gleixner wrote:
> > 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.
> 
> What's wrong with having an interrupt balancer tied to the scheduler
> which optimistically tries to avoid interrupting nohz/isolated/idle
> cpus?

Such an interrupt balancer would be a good thing, but I don't believe
that it will be sufficient.

> >  - The timer issue is mostly solved by the existing nohz stuff
> >    (plus/minus the few bugs in there).
> 
> Its not.. if you create an isolated domain there's no way to expel
> existing timers from there.

OK, I'll bite...  Why not just use CPU hotplug to expel the timers?

(Sorry, but you just can't expect me to pass that one up!)

> >  - The other details (silly IPIs) and cross CPU timer arming) are way
> >    easier to solve by a proper prohibitive state than by chasing that
> >    nonsense all over the tree forever. 
> 
> But we need to solve all that without a prohibitibe state anyway for the
> isolation stuff to be useful.

I bet that we will end up having to do both.

							Thanx, Paul

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