Re: [patch 2/3] KVM: x86: KVM_HC_RT_PRIO hypercall (host-side)

From: Marcelo Tosatti
Date: Thu Sep 21 2017 - 21:08:43 EST

On Thu, Sep 21, 2017 at 03:49:33PM +0200, Paolo Bonzini wrote:
> On 21/09/2017 15:32, Konrad Rzeszutek Wilk wrote:
> > So the guest can change the scheduling decisions at the host level?
> > And the host HAS to follow it? There is no policy override for the
> > host to say - nah, not going to do it?

In that case the host should not even configure the guest with this
option (this is QEMU's 'enable-rt-fifo-hc' option).

> > Also wouldn't the guest want to always be at SCHED_FIFO? [I am thinking
> > of a guest admin who wants all the CPU resources he can get]

No. Because in the following code, executed by the housekeeping vCPU
running at constant SCHED_FIFO priority:

1. Start disk I/O.
2. busy spin

With the emulator thread sharing the same pCPU with the housekeeping
vCPU, the emulator thread (which runs at SCHED_NORMAL), will never
be scheduled in in place of the vcpu thread at SCHED_FIFO.

This causes a hang.

> Yeah, I do not understand why there should be a housekeeping VCPU that
> is running at SCHED_NORMAL. If it hurts, don't do it...

Hope explanation above makes sense (in fact, it was you who pointed
out SCHED_FIFO should not be constant on the housekeeping vCPU,
when sharing pCPU with emulator thread at SCHED_NORMAL).