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).
