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

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Date:	Wed, 18 May 2016 16:45:26 +0100
From:	David Vrabel <david.vrabel@...rix.com>
To:	Juergen Gross <jgross@...e.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.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 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. :-)
> 
>> 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?).

David