On Mon, 2019-07-29 at 17:47 +0300, Andrii Anisov wrote:
> On 26.07.19 15:14, Juergen Gross wrote:
> > > > 
> > Tasklets are sometimes used to perform async actions which can't be
> > done
> > in guest vcpu context.
> > [...]
> > 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.
> 
No, stop_machine() is not about a VM being shutdown/killed/stopped.
It's, let's say, about all the (physical) CPU in the host having to do
something, in a coordinated way.

Check the comment in xen/include/xen/stop_machine.h

> > 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?
> 
It's possible, I guess, that we can implement some of the things that
are done in tasklets (either stop_machine() or something else)
differently, and in a way that is less disruptive for scheduling and
determinism.

But, if it is, it's not going to be as easy as <<let's run tasklet at
domain priority, and be done with it>>, I'm afraid. :-(

Regards
-- 
Dario Faggioli, Ph.D
http://about.me/dario.faggioli
Virtualization Software Engineer
SUSE Labs, SUSE https://www.suse.com/
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