linux-kernel - Re: [PATCH] xen: add steal

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Date:	Wed, 18 May 2016 12:13:51 -0400
From:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:	Juergen Gross <jgross@...e.com>,
	David Vrabel <david.vrabel@...rix.com>,
	xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org
Cc:	sstabellini@...nel.org
Subject: Re: [PATCH] xen: add steal_clock support on x86

On 05/18/2016 12:00 PM, Juergen Gross wrote:
> On 18/05/16 17:53, Boris Ostrovsky wrote:
>> On 05/18/2016 11:45 AM, David Vrabel wrote:
>>> On 18/05/16 16:42, Juergen Gross wrote:
>>>> On 18/05/16 17:25, Boris Ostrovsky wrote:
>>>>> On 05/18/2016 10:53 AM, Juergen Gross wrote:
>>>>>> On 18/05/16 16:46, Boris Ostrovsky wrote:
>>>>>>> On 05/18/2016 08:15 AM, Juergen Gross wrote:
>>>>>>>>  }
>>>>>>>>  
>>>>>>>> +void __init xen_time_setup_guest(void)
>>>>>>>> +{
>>>>>>>> +	pv_time_ops.steal_clock = xen_steal_clock;
>>>>>>>> +
>>>>>>>> +	static_key_slow_inc(&paravirt_steal_enabled);
>>>>>>>> +	/*
>>>>>>>> +	 * We can't set paravirt_steal_rq_enabled as this would require the
>>>>>>>> +	 * capability to read another cpu's runstate info.
>>>>>>>> +	 */
>>>>>>>> +}
>>>>>>> Won't we be accounting for stolen cycles twice now --- once from
>>>>>>> steal_account_process_tick()->steal_clock() and second time from
>>>>>>> do_stolen_accounting()?
>>>>>> Uuh, yes.
>>>>>>
>>>>>> I guess I should rip do_stolen_accounting() out, too? 
>>>>> I don't think PARAVIRT_TIME_ACCOUNTING is always selected for Xen. If
>>>> This is easy to accomplish. :-)
>>
>> I looked at KVM code (PARAVIRT_TIME_ACCOUNTING is not selected there
>> neither) and in their case that's presumably because stealing accounting
>> is a CPUID bit, i.e. it might not be supported. In Xen case we always
>> have this interface.
> So they added it later and the default is to keep the old behavior.
>
>>>>> that's indeed the case then we should ifndef do_stolen_accounting(). Or
>>>>> maybe check for paravirt_steal_enabled.
>>>> Is this really a sensible thing to do? There is a mechanism used by KVM
>>>> to do the stolen accounting. I think we should use it instead of having
>>>> a second implementation doing the same thing in case the generic one
>>>> isn't enabled.
>>> I agree.
>>>
>>> Although I don't think selecting PARAVIRT_TIME_ACC' is necessary -- I
>>> don't think it's essential (or is it?).
>> Looks like it's useful only if paravirt_steal_rq_enabled, which we don't
>> support yet.
> I think the patch is still useful. It is reducing code size and
> it is removing arch-specific Xen-hack(s). With the patch Xen's
> solution for arm and x86 is common and the same as for KVM. Adding
> paravirt_steal_rq_enabled later will be much easier as only one
> function needs substantial modification.

I am not arguing against having a patch that will remove
do_stolen_accounting(). I was responding to David's statement about
whether we need to select CONFIG_PARAVIRT_TIME_ACCOUNTING, and I am not
sure this is necessary since steal_account_process_tick() (that will
take case of things that do_stolen_accounting() currently does) doesn't
need it.

(And if it is indeed needed --- can we have Xen's Kconfig select it
instead of "default y if XEN" ?)

-boris