On Wed, 02 Apr 2014 10:15:29 -0700 Andi Kleen <[email protected]> wrote:
> Igor Mammedov <[email protected]> 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 > > -- > [email protected] -- Speaking for myself only -- Regards, Igor -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
