Re: Null scheduler and vwfi native problem

["Jürgen Groß" <jgross@suse.com>]

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On 03.02.21 10:19, Julien Grall wrote:
> Hi,
> 
> On 03/02/2021 07:31, Dario Faggioli wrote:
>> On Tue, 2021-02-02 at 15:23 +0000, Julien Grall wrote:
>>> In reality, it is probably still too early as a pCPU can be
>>> considered
>>> quiesced until a call to rcu_lock*() (such rcu_lock_domain()).
>>>
>> Well, yes, in theory, we could track down which is the first RCU read
>> side crit. section on this path, and put the call right before that (if
>> I understood what you mean).
> 
> Oh, that's not what I meant. This will indeed be far more complex than I 
> originally had in mind.
> 
> AFAIU, the RCU uses critical section to protect data. So the "entering" 
> could be used as "the pCPU is not quiesced" and "exiting" could be used 
> as "the pCPU is quiesced".
> 
> The concern with my approach is we would need to make sure that Xen 
> correctly uses the rcu helpers. I know Juergen worked on that recently, 
> but I don't know whether this is fully complete.

I think it is complete, but I can't be sure, of course.

One bit missing (for catching some wrong uses of the helpers) is this
patch:

https://lists.xen.org/archives/html/xen-devel/2020-03/msg01759.html

I don't remember why it hasn't been taken, but I think there was a
specific reason for that.


Juergen

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