Re: [PATCH v7 15/17] vmx: VT-d posted-interrupt core logic handling

[George Dunlap <george.dunlap@citrix.com>]

On 09/22/2015 06:10 AM, Wu, Feng wrote:
> 
> 
>> -----Original Message-----
>> From: Dario Faggioli [mailto:dario.faggioli@citrix.com]
>> Sent: Monday, September 21, 2015 10:12 PM
>> To: Wu, Feng; George Dunlap
>> Cc: xen-devel@lists.xen.org; Tian, Kevin; Keir Fraser; George Dunlap; Andrew
>> Cooper; Jan Beulich
>> Subject: Re: [Xen-devel] [PATCH v7 15/17] vmx: VT-d posted-interrupt core logic
>> handling
>>
>> On Mon, 2015-09-21 at 13:50 +0000, Wu, Feng wrote:
>>>
>>>> -----Original Message-----
>>>> From: Dario Faggioli [mailto:dario.faggioli@citrix.com]
>>
>>>> Note that, in case of preemptions, we are switching from a non-idle
>>>> vcpu to another, non-idle, vcpu, so lazy context switching to the
>>>> idle
>>>> vcpu should not have much to do with this case...
>>>
>>> So do you mean in preemptions, we cannot switch from non-idle to idle
>>> or
>>> Idle to non-idle, i.e, we can only switch from non-idle to non-idle?
>>>
>> That's what I meant. It's the definition of 'preemption' and of 'idle
>> task/vcpu', AFICT. I mean, the idle vcpu has the lowest possible
>> priority ever, so it can't really preempt anyone.
>>
>> OTOH, if the idle vcpu is running, that means there weren't any active
>> vcpus because, e.g., all were blocked; therefore, any active vcpu
>> wanting to run would have to wake up (and hence go throught the proper
>> wake up logic) before trying to preempt idle.
>>
>> There is one possible caveat: tasklets. In fact (as you can check
>> yourself by looking, in the code, for tasklet_work_scheduled), it is
>> possible that we force the idle vcpu to execute, even when we have
>> active and runnable vcpus, to let it process tasklets. I'm not really
>> sure whether this could be a problem for you or not, can you have a
>> look (and/or, a try) and report back?
> 
> Thanks for your information about the tasklets part, it is very important,
> consider the following scenario:
> 
> non-idle vCPUA --> idle (tasklet) --> non-idle vCPUA
> 
> The above context switch will use the lazy context switch and cannot be
> covered in __context_switch(), we need to change SN's state during the
> "non-idle to idle" and "idle to non-idle" transition, so that means we need
> add the PI related logic in context_switch() instead of only in __context_switch().
> 
> Besides that, it is more robust to do the PI switch in context_switch() which
> can cover lazy context switch. Maybe in future, there are some other
> feature needing execute task _inside_ an idle context, and it may introduce
> some issues due to no PI state transition, and it is a little hard to debug.

So a transition like the above could happen in the case of a cpu that's
gone offline (e.g., to allow the devicemodel to handle an IO); or, as
you say, if we're doing urgent work in a tasklet such that it preempts a
running task.

One option would be to just ignore this -- in which case we would get
spurious PI interrupts, but no other major issues, right?

But if we want to avoid spurious PI interrupts when running idle, then
yes, we need *some* kind of a hook on the lazy context switch path.

/me does some more thinking...

 -George