On Wed, Jul 28, 2021 at 04:36:58PM +0900, Suleiman Souhlal wrote: > Hello, > > This series attempts to solve some issues that arise from > having some VCPUs be real-time while others aren't. > > We are trying to play media inside a VM on a desktop environment > (Chromebooks), which requires us to have some tasks in the guest > be serviced at real-time priority on the host so that the media > can be played smoothly. > > To achieve this, we give a VCPU real-time priority on the host > and use isolcpus= to ensure that only designated tasks are allowed > to run on the RT VCPU. WTH do you need isolcpus for that? What's wrong with cpusets? > In order to avoid priority inversions (for example when the RT > VCPU preempts a non-RT that's holding a lock that it wants to > acquire), we dedicate a host core to the RT vcpu: Only the RT > VCPU is allowed to run on that CPU, while all the other non-RT > cores run on all the other host CPUs. > > This approach works on machines that have a large enough number > of CPUs where it's possible to dedicate a whole CPU for this, > but we also have machines that only have 2 CPUs and doing this > on those is too costly. > > This patch series makes it possible to have a RT VCPU without > having to dedicate a whole host core for it. > It does this by making it so that non-RT VCPUs can't be > preempted if they are in a critical section, which we > approximate as having interrupts disabled or non-zero > preempt_count. Once the VCPU is found to not be in a critical > section anymore, it will give up the CPU. > There measures to ensure that preemption isn't delayed too > many times. > > (I realize that the hooks in the scheduler aren't very > tasteful, but I couldn't figure out a better way. > SVM support will be added when sending the patch for > inclusion.) > > Feedback or alternatives are appreciated. This is disguisting and completely wrecks the host scheduling. You're placing guest over host, that's fundamentally wrong. NAK! If you want co-ordinated RT scheduling, look at paravirtualized deadline scheduling.
