On 24/02/2017 14:04, Marcelo Tosatti wrote: >>>>> Whats the current usecase, or forseeable future usecase, for save/restore >>>>> across preemption again? (which would validate the broken by design >>>>> claim). >>>> Stop a guest that is using cpufreq, start a guest that is not using it. >>>> The second guest's performance now depends on the state that the first >>>> guest left in cpufreq. >>> Nothing forbids the host to implement switching with the >>> current hypercall interface: all you need is a scheduler >>> hook. >> Can it be done in vcpu_load/vcpu_put? But you still would have two >> components (KVM and sysfs) potentially fighting over the frequency, and >> that's still a bit ugly. > > Change the frequency at vcpu_load/vcpu_put? Yes: call into > cpufreq-userspace. But there is no notion of "per-task frequency" on the > Linux kernel (which was the starting point of this subthread). There isn't, but this patchset is providing a direct path from a task to cpufreq-userspace. This is as close as you can get to a per-task frequency. > But if you configure all CPUs in the system as cpufreq-userspace, > then some other (userspace program) has to decide the frequency > for the other CPUs. > > Which agent would do that and why? Thats why i initially said "whats the > usecase". You could just pin them at the highest non-TurboBoost frequency until a guest runs. That's assuming that they are idle and, because of isol_cpus/nohz_full, they would be almost always in deep C state anyway. Paolo
