KVM: Never start grow vCPU halt_poll_ns from value below halt_poll_ns_grow_start

grow_halt_poll_ns() have a strange behaviour in case
(vcpu->halt_poll_ns != 0) &&
(vcpu->halt_poll_ns < halt_poll_ns_grow_start).

In this case, vcpu->halt_poll_ns will be multiplied by grow factor
(halt_poll_ns_grow) which will require several grow iteration in order
to reach a value bigger than halt_poll_ns_grow_start.
This means that growing vcpu->halt_poll_ns from value of 0 is slower
than growing it from a positive value less than halt_poll_ns_grow_start.
Which is misleading and inaccurate.

Fix issue by changing grow_halt_poll_ns() to set vcpu->halt_poll_ns
to halt_poll_ns_grow_start in any case that
(vcpu->halt_poll_ns < halt_poll_ns_grow_start).
Regardless if vcpu->halt_poll_ns is 0.

use READ_ONCE to get a consistent number for all cases.

Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Liran Alon <liran.alon@oracle.com>
Signed-off-by: Nir Weiner <nir.weiner@oracle.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
index 29ffc99..062f3c9 100644 (file)
--- a/arch/powerpc/kvm/book3s_hv.c
+++ b/arch/powerpc/kvm/book3s_hv.c
@@ -3634,10 +3634,9 @@ static void grow_halt_poll_ns(struct kvmppc_vcore *vc)
        if (!halt_poll_ns_grow)
                return;
 
-       if (vc->halt_poll_ns == 0)
+       vc->halt_poll_ns *= halt_poll_ns_grow;
+       if (vc->halt_poll_ns < halt_poll_ns_grow_start)
                vc->halt_poll_ns = halt_poll_ns_grow_start;
-       else
-               vc->halt_poll_ns *= halt_poll_ns_grow;
 }
 
 static void shrink_halt_poll_ns(struct kvmppc_vcore *vc)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index ae818d2..5087cf7 100644 (file)
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -2189,17 +2189,17 @@ void kvm_sigset_deactivate(struct kvm_vcpu *vcpu)
 
 static void grow_halt_poll_ns(struct kvm_vcpu *vcpu)
 {
-       unsigned int old, val, grow;
+       unsigned int old, val, grow, grow_start;
 
        old = val = vcpu->halt_poll_ns;
+       grow_start = READ_ONCE(halt_poll_ns_grow_start);
        grow = READ_ONCE(halt_poll_ns_grow);
        if (!grow)
                goto out;
 
-       if (val == 0)
-               val = halt_poll_ns_grow_start;
-       else
-               val *= grow;
+       val *= grow;
+       if (val < grow_start)
+               val = grow_start;
 
        if (val > halt_poll_ns)
                val = halt_poll_ns;