> On Thu, May 19, 2016 at 06:44:58PM -0700, Yunhong Jiang wrote: > > The VMX-preemption timer is a feature on VMX, it counts down, from the > > value loaded by VM entry, in VMX nonroot operation. When the timer > > counts down to zero, it stops counting down and a VM exit occurs. > > > > The VMX preemption timer for tsc deadline timer virtualization. The > > VMX preemption timer is armed when the vCPU is running, and a VMExit > > will happen if the virtual TSC deadline timer expires. > > > > When the vCPU thread is scheduled out, the tsc deadline timer > > virtualization will be switched to use the current solution, i.e. use > > the timer for it. It's switched back to VMX preemption timer when the > > vCPU thread is scheduled int. > > > > This solution replace the complex OS's hrtimer system, and also the > > host timer interrupt handling cost, with a preemption_timer VMexit. It > > fits well for some NFV usage scenario, when the vCPU is bound to a > > pCPU and the pCPU is isolated, or some similar scenarioes. > > > > However, it possibly has impact if the vCPU thread is scheduled in/out > > very frequently, because it switches from/to the hrtimer emulation a > > lot. A module parameter is provided to turn it on or off. > > > > Signed-off-by: Yunhong Jiang <yunhong.jiang@xxxxxxxxx> > > Hi Yunhong Jiang, > > This adds cost to the VM-exit and VM-entry paths (additional > instructions and i-cache pressure). Also it adds cost to > kvm_sched_out. For now my review limited itself to making the code nicer without touching the overall design too much. I'm confident that we can reduce it to a dozen instructions on vmentry and only pay the cost of hrtimer_start on failed HLT polls (enabling the hrtimer only before going to sleep). Assuming that device interrupts are delivered while the guest is running or during a successful HLT polls, that would be very rare. > What is the benefit the switch from external interrupt to VMX preemption > timer brings? hrtimer_start/hrtimer_cancel do show up in profiles for dynticks guests. Since they touch a red-black tree, it shouldn't be hard to outperform them. Paolo -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
