linux-kernel - Re: [Xen-devel] [PATCH 1/3] arm_arch_timer: introduce arch_timer_stolen

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Date:	Mon, 06 May 2013 10:55:39 -0400
From:	Christopher Covington <cov@...eaurora.org>
To:	Stefano Stabellini <stefano.stabellini@...citrix.com>
CC:	Marc Zyngier <marc.zyngier@....com>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	"linux@....linux.org.uk" <linux@....linux.org.uk>,
	Ian Campbell <Ian.Campbell@...rix.com>,
	Arnd Bergmann <arnd@...db.de>,
	"konrad.wilk@...cle.com" <konrad.wilk@...cle.com>,
	Catalin Marinas <Catalin.Marinas@....com>,
	Will Deacon <Will.Deacon@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"john.stultz@...aro.org" <john.stultz@...aro.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [Xen-devel] [PATCH 1/3] arm_arch_timer: introduce arch_timer_stolen_ticks

On 05/05/2013 12:47 PM, Stefano Stabellini wrote:
> On Fri, 3 May 2013, Marc Zyngier wrote:
>> On 03/05/13 11:43, Stefano Stabellini wrote:
>>> On Thu, 2 May 2013, Christopher Covington wrote:
>>>>> So the virtual timer should appear to have been running even while time
>>>>> is being stolen and therefore stolen time needs to be accounted via some
>>>>> other means.
>>>>
>>>> Something that's not currently obvious to me is that given that the stolen
>>>> cycle accounting should be done, what makes the architected timer interrupt
>>>> handler the ideal place to do it?
>>>
>>> That is a good question and I would appreciate suggestions to improve
>>> the patch.
>>>
>>> Given that Xen x86 and ia64 does stolen time accounting from the timer
>>> interrupt handler:
>>>
>>> arch/x86/xen/time.c:xen_timer_interrupt
>>> arch/ia64/kernel/time.c:timer_interrupt
>>>
>>> and given that the arch_timer is the only timer used by Xen on ARM and
>>> that it includes a virt_timer that is made on purpose to be used by
>>> virtual machines, I thought that it might be a good place for it.
>>>
>>> I also thought that doing it this way, KVM should be able to reuse the
>>> same hook.
>>
>> Indeed. I just need to understand how time stealing works there ;-).
>>
>> Now, KVM is not necessarily limited to arch_timers, and we've run KVM
>> using a QEMU-provided timer in the past. Can you think of a more generic
>> location for this hook? Possibly something that would satisfy the
>> requirements of other architectures while we're at it?
> 
> Probably the best option would be to reuse
> 
> kernel/sched/cputime.c:steal_account_process_tick
> 
> but that also means introducing CONFIG_PARAVIRT on arm.
> I am up for that, what do you think?

That sounds like a good move to me.

Christopher

-- 
Employee of Qualcomm Innovation Center, Inc.
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