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KVM: Never start grow vCPU halt_poll_ns from value below halt_poll_ns_grow_start

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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>
This commit is contained in:
Nir Weiner 2019-01-27 12:17:16 +02:00 committed by Paolo Bonzini
parent 49113d360b
commit dee339b5c1
2 changed files with 7 additions and 8 deletions
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5
arch/powerpc/kvm/book3s_hv.c
View File

@ -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)

10
virt/kvm/kvm_main.c
View File

@ -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;