On Friday, February 24, 2017 04:34:52 PM Paolo Bonzini wrote: > > 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. Good discussion so far, but it should be happening on the linux-pm list. Would it be possible to repost the patches with a CC to linux-pm? Thanks, Rafael
