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From: George Dunlap <george.dunlap@citrix.com>
To: "Wu, Feng" <feng.wu@intel.com>,
	Dario Faggioli <dario.faggioli@citrix.com>
Cc: "Tian, Kevin" <kevin.tian@intel.com>, Keir Fraser <keir@xen.org>,
	George Dunlap <george.dunlap@eu.citrix.com>,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	"xen-devel@lists.xen.org" <xen-devel@lists.xen.org>,
	Jan Beulich <jbeulich@suse.com>
Subject: Re: [PATCH v7 15/17] vmx: VT-d posted-interrupt core logic handling
Date: Tue, 22 Sep 2015 11:43:09 +0100	[thread overview]
Message-ID: <560130BD.6090700@citrix.com> (raw)
In-Reply-To: <E959C4978C3B6342920538CF579893F002726954@SHSMSX104.ccr.corp.intel.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

  reply	other threads:[~2015-09-22 10:43 UTC|newest]

Thread overview: 56+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-09-21  5:08 [PATCH v7 15/17] vmx: VT-d posted-interrupt core logic handling Wu, Feng
2015-09-21  9:18 ` George Dunlap
2015-09-21 11:59   ` Wu, Feng
2015-09-21 13:31     ` Dario Faggioli
2015-09-21 13:50       ` Wu, Feng
2015-09-21 14:11         ` Dario Faggioli
2015-09-22  5:10           ` Wu, Feng
2015-09-22 10:43             ` George Dunlap [this message]
2015-09-22 10:46               ` George Dunlap
2015-09-22 13:25                 ` Wu, Feng
2015-09-22 13:40                   ` Dario Faggioli
2015-09-22 13:52                     ` Wu, Feng
2015-09-22 14:15                       ` George Dunlap
2015-09-22 14:38                         ` Dario Faggioli
2015-09-23  5:52                           ` Wu, Feng
2015-09-23  7:59                             ` Dario Faggioli
2015-09-23  8:11                               ` Wu, Feng
2015-09-22 14:28                   ` George Dunlap
2015-09-23  5:37                     ` Wu, Feng
  -- strict thread matches above, loose matches on Subject: below --
2015-09-21  5:09 Wu, Feng
2015-09-21  9:54 ` George Dunlap
2015-09-21 12:22   ` Wu, Feng
2015-09-21 14:24     ` Dario Faggioli
2015-09-22  7:19       ` Wu, Feng
2015-09-22  8:59         ` Jan Beulich
2015-09-22 13:40           ` Wu, Feng
2015-09-22 14:01             ` Jan Beulich
2015-09-23  9:44               ` George Dunlap
2015-09-23 12:35                 ` Wu, Feng
2015-09-23 15:25                   ` George Dunlap
2015-09-23 15:38                     ` Jan Beulich
2015-09-24  1:50                     ` Wu, Feng
2015-09-24  3:35                       ` Dario Faggioli
2015-09-24  7:51                       ` Jan Beulich
2015-09-24  8:03                         ` Wu, Feng
2015-09-22 10:26         ` George Dunlap
2015-09-23  6:35           ` Wu, Feng
2015-09-23  7:11             ` Dario Faggioli
2015-09-23  7:20               ` Wu, Feng
2015-09-11  8:28 [PATCH v7 00/17] Add VT-d Posted-Interrupts support Feng Wu
2015-09-11  8:29 ` [PATCH v7 15/17] vmx: VT-d posted-interrupt core logic handling Feng Wu
2015-09-16 16:00   ` Dario Faggioli
2015-09-16 17:18   ` Dario Faggioli
2015-09-16 18:05     ` Dario Faggioli
2015-09-17  8:00     ` Wu, Feng
2015-09-17  8:48       ` Dario Faggioli
2015-09-17  9:16         ` Wu, Feng
2015-09-17  9:38         ` George Dunlap
2015-09-17  9:39           ` George Dunlap
2015-09-17 11:44           ` George Dunlap
2015-09-17 12:40             ` Dario Faggioli
2015-09-17 14:30               ` George Dunlap
2015-09-17 16:36                 ` Dario Faggioli
2015-09-18  6:27                 ` Jan Beulich
2015-09-18  9:22                   ` Dario Faggioli
2015-09-18 14:31                     ` George Dunlap
2015-09-18 14:34                       ` George Dunlap

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