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

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Date:	Wed, 6 Jun 2012 08:49:45 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Arjan van de Ven <arjan@...ux.intel.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	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>,
	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 Wed, Jun 06, 2012 at 08:23:54AM -0700, Arjan van de Ven wrote:
> On 6/6/2012 7:41 AM, Paul E. McKenney wrote:
> > On Wed, Jun 06, 2012 at 10:43:43AM +0200, Peter Zijlstra wrote:
> >> On Tue, 2012-06-05 at 15:12 -0700, Paul E. McKenney wrote:
> >>>> What I can't see is the isolated functional, aside from the above
> >>>> mentioned things, that's not strictly a per-cpu property, we can have a
> >>>> group that's isolated from the rest but not from each other.
> >>>
> >>> I suspect that Thomas is thinking that the CPU is so idle that it no
> >>> longer has to participate in TLB invalidation or RCU.  (Thomas will
> >>> correct me if I am confused.)  But Peter, is that the level of idle
> >>> you are thinking of? 
> >>
> >> No, we're talking about isolated, so its very much running something.
> > 
> > From what I can see, if the CPU is running something, this is Thomas's
> > "Isolated functional" state rather than his "Isolated idle" state.
> > The isolated-idle state should not need to participate in TLB invalidation
> > or RCU, so that the CPU never ever needs to wake up while in the
> > isolated-idle state.
> 
> btw TLB invalidation I think is a red herring in this discussion
> (other than "global PTEs" kind of kernel pte changes);
> at least on x86 this is not happening for a long time; if a CPU is
> really idle (which means the CPU internally flushes the tlbs anyway),
> Linux also switches to the kernel PTE set so there's no need for a flush
> later on.

Right, as I understand it, only unmappings in the kernel address space
would need to IPI an idle CPU.  But this is still a source of IPIs that
could wake up the CPU, correct?

							Thanx, Paul

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