Re: [PROPOSAL] Doing work in idle-vcpu context

[Juergen Gross <juergen.gross@ts.fujitsu.com>]

Keir Fraser wrote:
> George, Yunhong, and others,
> 
> So, it seems that runing stop_machine_run(), and now
> continue_hypercall_on_cpu(), in softirq context is a bit of a problem.
> Because the softirq can stop the currently-running vcpu from being
> descheduled we can end up with subtle deadlocks. For example, with s_m_r()
> we try to rendezvous all cpus in softirq context -- we can have CPU A enter
> the softirq interrupting VCPU X, meanwhile VCPU Y on CPU B is spinning
> trying to pause VCPU X. Hence CPU B doesn't get into softirq, and so CPU A
> never leaves it, and we have deadlock.
> 
> There are various possible solutions to this, but one of the architecturally
> neatest would be to run the s_m_r() and c_h_o_c() work in a
> 'Linux-workqueue' type of environment -- i.e., in a proper non-guest vcpu
> context. Rather than introducing the whole kthread concept into Xen, one
> possibility would be to schedule this work on the idle vcpus -- effectively
> promoting idle vcpus to a more general kind of 'Xen worker vcpu' whose job
> can include running the idle loop.
> 
> One bit of mechanism this would require is the ability to bump the idle vcpu
> priority up - preferably to 'max' priority forcing it to run next until we
> return it to idle/lowest priority. George: how hard would such a mechanism
> be to implement do you think?
> 
> More generally: what do people think of this idea?

Sounds a little bit like my original proposal for the change of c_h_o_c():
Introduce a "hypervisor domain" for stuff like this.
I still think, this would be cleaner. The hypervisor vcpus would run with
high priority and they could block without rising major problems.


Juergen

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Juergen Gross                 Principal Developer Operating Systems
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