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

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Message-ID: <87zk8iioam.fsf@rustcorp.com.au>
Date:	Tue, 05 Jun 2012 10:10:33 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>
Cc:	Fenghua Yu <fenghua.yu@...el.com>, Ingo Molnar <mingo@...e.hu>,
	H Peter Anvin <hpa@...or.com>,
	Suresh B Siddha <suresh.b.siddha@...el.com>,
	Tony Luck <tony.luck@...el.com>,
	Asit K Mallick <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 Mon, 04 Jun 2012 22:33:21 +0200, Peter Zijlstra <peterz@...radead.org> wrote:
> On Mon, 2012-06-04 at 22:11 +0200, Thomas Gleixner wrote:
> 
> > I understand what you are trying to do, though I completely disagree
> > with the solution.
> > 
> > The main problem of the current hotplug code is that it is an all or
> > nothing approach. You have to tear down the whole thing completely
> > instead of just taking it out of the usable set of cpus.
> > 
> > I'm working on a proper state machine driven online/offline sequence,
> > where you can put the cpu into an intermediate state which avoids
> > bringing it down completely. This is enough to get the full
> > powersaving benefits w/o having to go through all the synchronization
> > states of a full online/offline. That will shorten the onlining time
> > of an previously offlined cpu to almost nothing.
> > 
> > I really want to avoid adding more bandaids to the hotplug code before
> > we have sorted out the existing horror.
> 
> Its far worse.. you shouldn't _ever_ care about hotplug latency unless
> you've got absolutely braindead hardware. We all now ARM has been
> particularly creative here, but is Intel now trying to trump ARM at
> stupid?

I disagree.  Deactivating a cpu for power saving is halfway to hotplug
anyway.  I'd rather unify the two cases, where we can specify how dead a
CPU should be, than have individual archs and boards do random hacks.

It also gives us a great excuse to audit and neaten various of the
hotplug cpu callbacks; most of the ones I've looked at have been racy :(

The ones which simply want to keep per-cpu stats can be given a nice
helper with two simple callbacks: one to empty stats for a going-away
cpu, and (maybe) one to restore them.

The per-cpu kthreads should no longer get torn down and recreated, and
doing it via a separate notifier function is ugly and error-prone.  My
plan is a "bool kthread_cpu_going(void)" and then a "void
kthread_cpu_can_go(void)", so kthreads can do:

        if (kthread_cpu_going()) {
                /* Do any cleanup we need. */
                ...

                /* This returns when CPU comes back. */
                kthread_cpu_can_go();
        }

Yeah, we should probably have the kthread exit inside
kthread_cpu_can_go() if they stop the kthread, but that's a detail.

Cheers,
Rusty.
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