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SubjectRe: [PATCH v2 1/5] x86: replace timeouts when booting secondary CPU with infinite wait loop
On Wed, 02 Apr 2014 10:15:29 -0700
Andi Kleen <andi@firstfloor.org> wrote:

> Igor Mammedov <imammedo@redhat.com> writes:
>
> > Hang is observed on virtual machines during CPU hotplug,
> > especially in big guests with many CPUs. (It reproducible
> > more often if host is over-committed).
> >
> > It happens because master CPU gives up waiting on
> > secondary CPU and allows it to run wild. As result
> > AP causes locking or crashing system. For example
> > as described here: https://lkml.org/lkml/2014/3/6/257
> >
> > If master CPU have sent STARTUP IPI successfully,
> > make it wait indefinitely till AP boots.
>
>
> But what happens on a real machine when the other CPU is dead?
One possible way to boot such machine would be to disable dead CPU
in kernel parameters.

> I've seen that. Kernel still boots. With your patch it would
> hang.
>
> I don't think you can do that. It needs to have some timeout.
> Maybe a longer or configurable one?
there were patch that tried to keep timeouts and 'gracefully'
cancel AP boot if master timed out on it.
https://lkml.org/lkml/2014/3/6/257

It's possible to keep timeouts in do_boot_cpu(), is setting
trampoline_status sufficient indication that AP is not dead
and worth waiting for?

than it could be rewritten like this:
if (!boot_error) {
boot_error = 1;
for (timeout = 0; timeout < 50000; timeout++) {
/* Wait till AP signals that it's ready to start initialization */
if (*trampoline_status == 0xA5A5A5A5) {
boot_error = 0;
/* allow AP to start initializing. */
cpumask_set_cpu(cpu, cpu_callout_mask);

/* wait till AP boots till cpu_callin_mask point */
while (cpumask_test_cpu(cpu, cpu_callin_mask))
schedule();

break; /* It has booted */
}
udelay(100);
}
}

it will provide timeout if AP is dead and still keep AP from running wild
if master CPU timed out on it.


>
> -Andi
>
> --
> ak@linux.intel.com -- Speaking for myself only


--
Regards,
Igor


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