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SubjectRe: [PATCH 2/2] mm: page allocator: Do not drain per-cpu lists via IPI from page allocator context
On Fri, Jan 20, 2012 at 03:16:58AM +0530, Srivatsa S. Bhat wrote:
> [Reinstating the original Cc list]
>
> On 01/19/2012 09:50 PM, Mel Gorman wrote:>
>
> > On a different x86-64 machines with an intel-specific MCE, I have
> > also noted that the value of num_online_cpus() can change while
> > stop_machine() is running.
>
>
> That is expected and intentional right? Meaning, it is during the
> stop_machine() thing itself that a CPU is actually taken offline.
> And at the same time, it is removed from the cpu_online_mask.
>

It's intentional sometimes and no others. The machine does halt
sometimes and stays there.

> On Intel boxes, essentially, the following gets executed on the dying
> CPU, as set up by the stop_machine stuff.
>
> __cpu_disable()
> native_cpu_disable()
> cpu_disable_common()
> remove_cpu_from_maps()
> set_cpu_online(cpu, false)
> ^^^^^^
> So, set_cpu_online will remove this CPU from the cpu_online_mask.
> And all this runs while still under the stop machine context.
> And this is exactly what we want right?
>

We don't want it to halt in stop_machine forever waiting on acknowledges
that are never received until the NMI handler fires.

> > This is sensitive to timing and part of
> > the problem seems to be due to cmci_rediscover() running without the
> > CPU hotplug mutex held. This is not related to the IPI mess and is
> > unrelated to memory pressure but is just to note that CPU hotplug in
> > general can be fragile in parts.
> >
>
>
> For the cmci_rediscover() part, I feel a simple get/put_online_cpus()
> around it should work.
>

Yeah, that's the first thing I tried first too. Doesn't work though.

--
Mel Gorman
SUSE Labs


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