On Wed, Feb 26, 2025, Fernand Sieber wrote: > On Tue, 2025-02-25 at 18:17 -0800, Sean Christopherson wrote: > > > In this RFC we introduce the concept of guest halted time to address > > > these concerns. Guest halted time (gtime_halted) accounts for cycles > > > spent in guest mode while the cpu is halted. gtime_halted relies on > > > measuring the mperf msr register (x86) around VM enter/exits to compute > > > the number of unhalted cycles; halted cycles are then derived from the > > > tsc difference minus the mperf difference. > > > > IMO, there are better ways to solve this than having KVM sample MPERF on > > every entry and exit. > > > > The kernel already samples APERF/MPREF on every tick and provides that > > information via /proc/cpuinfo, just use that. If your userspace is unable > > to use /proc/cpuinfo or similar, that needs to be explained. > > If I understand correctly what you are suggesting is to have userspace > regularly sampling these values to detect the most idle CPUs and then > use CPU affinity to repin housekeeping tasks to these. While it's > possible this essentially requires to implement another scheduling > layer in userspace through constant re-pinning of tasks. This also > requires to constantly identify the full set of tasks that can induce > undesirable overhead so that they can be pinned accordingly. For these > reasons we would rather want the logic to be implemented directly in > the scheduler. > > > And if you're running vCPUs on tickless CPUs, and you're doing HLT/MWAIT > > passthrough, *and* you want to schedule other tasks on those CPUs, then IMO > > you're abusing all of those things and it's not KVM's problem to solve, > > especially now that sched_ext is a thing. > > We are running vCPUs with ticks, the rest of your observations are > correct. If there's a host tick, why do you need KVM's help to make scheduling decisions? It sounds like what you want is a scheduler that is primarily driven by MPERF (and APERF?), and sched_tick() => arch_scale_freq_tick() already knows about MPERF.
