• Suraj Jitindar Singh's avatar
    KVM: PPC: Book3S HV: Implement halt polling · 0cda69dd
    Suraj Jitindar Singh authored
    This patch introduces new halt polling functionality into the kvm_hv kernel
    module. When a vcore is idle it will poll for some period of time before
    scheduling itself out.
    
    When all of the runnable vcpus on a vcore have ceded (and thus the vcore is
    idle) we schedule ourselves out to allow something else to run. In the
    event that we need to wake up very quickly (for example an interrupt
    arrives), we are required to wait until we get scheduled again.
    
    Implement halt polling so that when a vcore is idle, and before scheduling
    ourselves, we poll for vcpus in the runnable_threads list which have
    pending exceptions or which leave the ceded state. If we poll successfully
    then we can get back into the guest very quickly without ever scheduling
    ourselves, otherwise we schedule ourselves out as before.
    
    There exists generic halt_polling code in virt/kvm_main.c, however on
    powerpc the polling conditions are different to the generic case. It would
    be nice if we could just implement an arch specific kvm_check_block()
    function, but there is still other arch specific things which need to be
    done for kvm_hv (for example manipulating vcore states) which means that a
    separate implementation is the best option.
    
    Testing of this patch with a TCP round robin test between two guests with
    virtio network interfaces has found a decrease in round trip time of ~15us
    on average. A performance gain is only seen when going out of and
    back into the guest often and quickly, otherwise there is no net benefit
    from the polling. The polling interval is adjusted such that when we are
    often scheduled out for long periods of time it is reduced, and when we
    often poll successfully it is increased. The rate at which the polling
    interval increases or decreases, and the maximum polling interval, can
    be set through module parameters.
    
    Based on the implementation in the generic kvm module by Wanpeng Li and
    Paolo Bonzini, and on direction from Paul Mackerras.
    Signed-off-by: default avatarSuraj Jitindar Singh <sjitindarsingh@gmail.com>
    Signed-off-by: default avatarPaul Mackerras <paulus@ozlabs.org>
    0cda69dd
kvm_book3s.h 10.7 KB