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
next prev parent 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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