On Wed, Jul 28, 2021 at 7:34 PM Marcelo Tosatti <mtosatti@xxxxxxxxxx> wrote: > > On Wed, Jul 28, 2021 at 10:10:31AM +0200, Peter Zijlstra 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? > > > > > 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. > > Peter, not sure what exactly are you thinking of? (to solve this > particular problem with pv deadline scheduling). > > Shouldnt it be possible to, through paravirt locks, boost the priority > of the non-RT vCPU (when locking fails in the -RT vCPU) ? Unfortunately paravirt locks doesn't work in this configuration because sched_yield() doesn't work across scheduling classes (non-RT vs RT). :-( -- Suleiman
