Hi Peter, On Wed, Jul 28, 2021 at 5:11 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > > 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? I regret mentioning isolcpus here. The patchset doesn't dictate how the guest is supposed to use RT. cpusets also work. > > 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. I understand the sentiment. For what it's worth, the patchset doesn't completely rely on a well-behaved guest: It only delays preemption a bounded number of times, after which it yields back no matter what. > NAK! > > If you want co-ordinated RT scheduling, look at paravirtualized deadline > scheduling. Thanks for the suggestion, I will look into it. -- Suleiman
