Re: [PATCH v5 7/8] mm: Only IPI CPUs to drain local pages if theyexist

[Paul E. McKenney]

On Sat, Jan 07, 2012 at 08:52:01AM -0800, Paul E. McKenney wrote:
> On Thu, Jan 05, 2012 at 04:17:39PM +0000, Mel Gorman wrote:
> > On Thu, Jan 05, 2012 at 02:40:11PM +0000, Russell King - ARM Linux wrote:
> > > On Thu, Jan 05, 2012 at 02:20:17PM +0000, Mel Gorman wrote:
> 
> [ . . . ]
> 
> > > I've been chasing that patch and getting no replies what so
> > > ever from folk like Peter, Thomas and Ingo.
> > > 
> > > The problem affects all IPI-raising functions, which mask with
> > > cpu_online_mask directly.
> > 
> > Actually, in one sense I'm glad to hear it because from my brief
> > poking around, I was having trouble understanding why we were always
> > safe from sending IPIs to CPUs in the process of being offlined.
> 
> The trick is to disable preemption (not interrupts!) across the IPI, which
> prevents CPU-hotplug's stop_machine() from running.  You also have to
> have checked that the CPU is online within this same preemption-disabled
> section of code.  This means that the outgoing CPU has to accept IPIs
> even after its CPU_DOWN_PREPARE notifier has been called -- right up
> to the stop_machine() call to take_cpu_down().

Of course, another trick is to hold the CPU-hotplug lock across the IPI,
but this is quite a bit more heavy-weight than disabling preemption.

							Thanx, Paul

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