linux-kernel - Re: [PATCH tip/core/rcu 02/20] x86: Use common outgoing-CPU-notification code

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Date:	Tue, 03 Mar 2015 17:06:50 -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 04:26 PM, Paul E. McKenney wrote:
> On Tue, Mar 03, 2015 at 03:13:07PM -0500, Boris Ostrovsky wrote:
>> 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.
> OK, so I have the following patch on top of my previous patch, which
> I will merge if testing goes well.  So if a CPU times out going offline,
> the above three functions will not be called, the "didn't die" message
> will be printed, and any future attempt to online that CPU will fail.
> Is that the correct semantics?

Yes.

I am not sure whether not ever onlining the CPU is the best outcome but 
then I don't think trying to online it again with all interrupts and 
such still set up will work well. And it's an improvement over what we 
have now anyway (with current code we may clean up things for a non-dead 
cpu).

Thanks.
-boris


>
> 							Thanx, Paul
>
> ------------------------------------------------------------------------
>
> diff --git a/arch/x86/xen/smp.c b/arch/x86/xen/smp.c
> index e2c7389c58c5..f2a06ff0614d 100644
> --- a/arch/x86/xen/smp.c
> +++ b/arch/x86/xen/smp.c
> @@ -508,12 +508,13 @@ static void xen_cpu_die(unsigned int cpu)
>   		schedule_timeout(HZ/10);
>   	}
>   
> -	(void)cpu_wait_death(cpu, 5);
> -	/* FIXME: Are the below calls really safe in case of timeout? */
> -
> -	xen_smp_intr_free(cpu);
> -	xen_uninit_lock_cpu(cpu);
> -	xen_teardown_timer(cpu);
> +	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);
> +	}
>   }
>   
>   static void xen_play_dead(void) /* used only with HOTPLUG_CPU */
>

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