From mboxrd@z Thu Jan 1 00:00:00 1970 From: Marcelo Tosatti Subject: Re: [patch 2/3] KVM: x86: KVM_HC_RT_PRIO hypercall (host-side) Date: Thu, 21 Sep 2017 22:08:13 -0300 Message-ID: <20170922010811.GA20133@amt.cnet> References: <20170921113835.031375194@redhat.com> <20170921114039.364395490@redhat.com> <20170921133212.GN26248@char.us.oracle.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Konrad Rzeszutek Wilk , kvm@vger.kernel.org, linux-kernel@vger.kernel.org To: Paolo Bonzini Return-path: Content-Disposition: inline In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org 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).