Messages in this thread | | | Date | Wed, 2 Apr 2014 23:29:56 +0200 | From | Igor Mammedov <> | Subject | Re: [PATCH v2 1/5] x86: replace timeouts when booting secondary CPU with infinite wait loop |
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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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