Re: [Xen-devel] [RFC 1/6] xen/arm: Re-enable interrupt later in the trap path

From: Julien Grall <julien.grall@arm.com>
To: Andrii Anisov <andrii.anisov@gmail.com>,
	Dario Faggioli <dfaggioli@suse.com>,
	xen-devel@lists.xenproject.org
Cc: Andrii Anisov <andrii_anisov@epam.com>,
	Stefano Stabellini <sstabellini@kernel.org>,
	Volodymyr Babchuk <Volodymyr_Babchuk@epam.com>
Subject: Re: [Xen-devel] [RFC 1/6] xen/arm: Re-enable interrupt later in the trap path
Date: Fri, 2 Aug 2019 10:15:05 +0100	[thread overview]
Message-ID: <189da8d7-dee8-18f8-fbbf-699761f0b656@arm.com> (raw)
In-Reply-To: <a71ddd42-173d-32be-0674-47818316232e@gmail.com>

Hi,

On 02/08/2019 08:50, Andrii Anisov wrote:
> Hello Dario
> 
> On 01.08.19 14:19, Dario Faggioli wrote:
>> On Thu, 2019-08-01 at 09:45 +0300, Andrii Anisov wrote:
>>> Hello Julien,
>>>
>>> On 30.07.19 23:10, Julien Grall wrote:
>>>
>>>>> In this series I think I need interrupts locked until I start
>>>>> time accounting for hypervisor. Time accounting is started by
>>>>> `tacc_head()` function. I prefer to have it called from C,
>>>>> because it is more convenient and obvious for those who are less
>>>>> familiar with the ARM code.
>>>
>>> Here is the question to you: what is the best place (and way) to
>>> start hypervisor time tracking?
>>>
>> This is actually quite an important question... And I'd like to throw
>> it back at you (Andrii)! :-D :-P :-)
> 
> At this series I start time accounting for hypervisor after the trap, before 
> interrupts being enabled. It is done for all traps except synchronous traps from 
> guest, what are hypecalls and io emulation. For synchronous traps, I start hyp 
> accounting after the guest's request has been served, and we start softirq 
> processing (actually all the stuff `leave_hypervisor_tail()` does). I believe it 
> should be so.
> 
>> In fact, I was about to ask myself something similar, such as, can we
>> take a bit of a step back and define:
>>
>> - what's the, let's say, accounting granularity we want? "Just" guest
>> and hypervisor? Or more fine grained?
> 
> As for me hypervisor/guest/idle is quite fine granularity for the beginning.
> Such approach might be enough to implement fair time accounting.
> Yet we might need something more sophisticated for RT scheduling. E.g. guest's 
> IRQ time tracking, to not let some guest to spam the system with its IRQs.
> 
>> - assuming we "just" want hypervisor and guest, which are the
>> events/turning points at which we should switch "timing accounting
>> bucket"? Can we make a list?
>> And I'd be fine for such list to be generic, in the first place (e.g.,
>> hypercall, IRQ, etc)... Then we'll turn the entries into actual
>> locations in the code, as a second step.
> 
> 
> I can make such a list, how it is done in this series:

 From the list below it is not clear what is the split between hypervisor time 
and guest time. See some of the examples below.

> 
> Guest -[async trap (IRQ)]-> Hyp : switch to hyp time accounting

Why all the interrupts should be accounted to the hypervisor? Why not accounting 
guest time for any interrupt routed to the current guest?

> Guest -[sync trap (hypercall, io emulation)]-> HYP : switch to hyp time 
> accounting *after* serving sync trap (hypercall, io emulation)

Why so? Some of the work after servicing an hypercall/IO emulation are still 
guest specific. For instance, we may update the hardware state for the guest 
(see vgic_sync_to_lrs()). We may also have defer work that take a long time (see 
check_for_pcpu_work()).

IHMO, the only part worth accounting to the hypervisor mode is 
check_for_pcpu_work().

Cheers,

-- 
Julien Grall

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