Re: [Xen-devel] [RFC 0/6] XEN scheduling hardening

[Andrii Anisov <andrii.anisov@xxxxxxxxx>]

Hello Juergen,

On 26.07.19 15:14, Juergen Gross wrote:
> > >   - How to avoid the absolute top priority of tasklets (what is obeyed
> > > by all
> > >     schedulers so far). Should idle vcpu be scheduled as the normal
> > > guest vcpus
> > >     (through queues, priorities, etc)?
> > Now, this is something to think about, and try to understand if
> > anything would break if we go for it. I mean, I see why you'd want to
> > do that, but tasklets and softirqs works the way they do, in Xen, since
> > when they were introduced, I believe.
> >
> > Therefore, even if there wouldn't be any subsystem explicitly relying
> > on the current behavior (which should be verified), I think we are at
> > high risk of breaking things, if we change.
> We'd break things IMO.
>
> Tasklets are sometimes used to perform async actions which can't be done
> in guest vcpu context.
OK, that stuff can not be done in the guest vcpu context. But why can't it be 
done with the guest's priority?

> Like switching a domain to shadow mode for L1TF
> mitigation,
Sorry I'm not really aware what L1TF mitigation is.
But

> or marshalling all cpus for stop_machine().
I think I faced some time ago. When fixed a noticeable lag in video playback at 
the moment of the other domain reboot.

> You don't want
> to be able to block tasklets, you want them to run as soon as possible.
Should it really be done in the way of breaking scheduling? What if the system 
has RT requirements?

--
Sincerely,
Andrii Anisov.

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