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Date:	Tue, 03 Mar 2015 15:13:07 -0500
From:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:	paulmck@...ux.vnet.ibm.com
CC:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
	dvhart@...ux.intel.com, fweisbec@...il.com, oleg@...hat.com,
	bobby.prani@...il.com, x86@...nel.org,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	David Vrabel <david.vrabel@...rix.com>,
	xen-devel@...ts.xenproject.org
Subject: Re: [PATCH tip/core/rcu 02/20] x86: Use common outgoing-CPU-notification
 code

On 03/03/2015 02:42 PM, Paul E. McKenney wrote:
> On Tue, Mar 03, 2015 at 02:17:24PM -0500, Boris Ostrovsky wrote:
>> On 03/03/2015 12:42 PM, Paul E. McKenney wrote:
>>>   }
>>> @@ -511,7 +508,8 @@ static void xen_cpu_die(unsigned int cpu)
>>>   		schedule_timeout(HZ/10);
>>>   	}
>>> -	cpu_die_common(cpu);
>>> +	(void)cpu_wait_death(cpu, 5);
>>> +	/* FIXME: Are the below calls really safe in case of timeout? */
>>
>>
>> Not for HVM guests (PV guests will only reach this point after
>> target cpu has been marked as down by the hypervisor).
>>
>> We need at least to have a message similar to what native_cpu_die()
>> prints on cpu_wait_death() failure. And I think we should not call
>> the two routines below (three, actually --- there is also
>> xen_teardown_timer() below, which is not part of the diff).
>>
>> -boris
>>
>>
>>>   	xen_smp_intr_free(cpu);
>>>   	xen_uninit_lock_cpu(cpu);
>
> So something like this, then?
>
> 	if (cpu_wait_death(cpu, 5)) {
> 		xen_smp_intr_free(cpu);
> 		xen_uninit_lock_cpu(cpu);
> 		xen_teardown_timer(cpu);
> 	}

	else
		pr_err("CPU %u didn't die...\n", cpu);


>
> Easy change for me to make if so!
>
> Or do I need some other check for HVM-vs.-PV guests, and, if so, what
> would that check be?  And also if so, is it OK to online a PV guest's
> CPU that timed out during its previous offline?


I believe PV VCPUs will always be CPU_DEAD by the time we get here since 
we are (indirectly) waiting for this in the loop at the beginning of 
xen_cpu_die():

'while (xen_pv_domain() && HYPERVISOR_vcpu_op(VCPUOP_is_up, cpu, NULL))' 
will exit only after 'HYPERVISOR_vcpu_op(VCPUOP_down, 
smp_processor_id()' in xen_play_dead(). Which happens after 
play_dead_common() has marked the cpu as CPU_DEAD.

So no test is needed.

Thanks.
-boris


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