linux-kernel - Re: [PATCH v2 1/5] x86: replace timeouts when booting secondary CPU with infinite wait loop

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Date:	Wed, 2 Apr 2014 23:29:56 +0200
From:	Igor Mammedov <imammedo@...hat.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com,
	hpa@...or.com, x86@...nel.org, bp@...e.de,
	paul.gortmaker@...driver.com, JBeulich@...e.com, prarit@...hat.com,
	drjones@...hat.com, toshi.kani@...com, riel@...hat.com,
	gong.chen@...ux.intel.com
Subject: Re: [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@...stfloor.org> wrote:

> Igor Mammedov <imammedo@...hat.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@...ux.intel.com -- Speaking for myself only


-- 
Regards,
  Igor
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