Default halt_poll_ns_shrink value of 0 always resets polling interval to 0 on an un-successful poll where vcpu wakeup is not received. This is mostly to avoid pointless polling for more number of shorter intervals. But disabled shrink assumes vcpu wakeup is less likely to be received in subsequent shorter polling intervals. Another side effect of 0 shrink value is that, even on a successful poll if total block time was greater than current polling interval, the polling interval starts over from 0 instead of shrinking by a factor. Enabling shrink with value of 2 allows the polling interval to gradually decrement in case of un-successful poll events as well. This gives a fair chance for successful polling events in subsequent polling intervals rather than resetting it to 0 and starting over from grow_start. Below kvm stat log snippet shows interleaved growth and shrinking of polling interval: 87162647182125: kvm_halt_poll_ns: vcpu 0: halt_poll_ns 10000 (grow 0) 87162647637763: kvm_halt_poll_ns: vcpu 0: halt_poll_ns 20000 (grow 10000) 87162649627943: kvm_halt_poll_ns: vcpu 0: halt_poll_ns 40000 (grow 20000) 87162650892407: kvm_halt_poll_ns: vcpu 0: halt_poll_ns 20000 (shrink 40000) 87162651540378: kvm_halt_poll_ns: vcpu 0: halt_poll_ns 40000 (grow 20000) 87162652276768: kvm_halt_poll_ns: vcpu 0: halt_poll_ns 20000 (shrink 40000) 87162652515037: kvm_halt_poll_ns: vcpu 0: halt_poll_ns 40000 (grow 20000) 87162653383787: kvm_halt_poll_ns: vcpu 0: halt_poll_ns 20000 (shrink 40000) 87162653627670: kvm_halt_poll_ns: vcpu 0: halt_poll_ns 10000 (shrink 20000) 87162653796321: kvm_halt_poll_ns: vcpu 0: halt_poll_ns 20000 (grow 10000) 87162656171645: kvm_halt_poll_ns: vcpu 0: halt_poll_ns 10000 (shrink 20000) 87162661607487: kvm_halt_poll_ns: vcpu 0: halt_poll_ns 0 (shrink 10000) Having both grow and shrink enabled creates a balance in polling interval growth and shrink behavior. Tests show improved successful polling attempt ratio which contribute to VM performance. Power penalty is quite negligible as shrunk polling intervals create bursts of very short durations. Performance assessment results show 3-6% improvements in CPU+GPU, Memory and Storage Android VM workloads whereas 5-9% improvement in average FPS of gaming VM workloads. Power penalty is below 1% where host OS is either idle or running a native workload having 2 VMs enabled. CPU/GPU intensive gaming workloads as well do not show any increased power overhead with shrink enabled. Co-developed-by: Rajendran Jaishankar <jaishankar.rajendran@xxxxxxxxx> Signed-off-by: Rajendran Jaishankar <jaishankar.rajendran@xxxxxxxxx> Signed-off-by: Parshuram Sangle <parshuram.sangle@xxxxxxxxx> --- Documentation/virt/kvm/halt-polling.rst | 2 +- virt/kvm/kvm_main.c | 4 ++-- 2 files changed, 3 insertions(+), 3 deletions(-) diff --git a/Documentation/virt/kvm/halt-polling.rst b/Documentation/virt/kvm/halt-polling.rst index c82a04b709b4..64f32a81133f 100644 --- a/Documentation/virt/kvm/halt-polling.rst +++ b/Documentation/virt/kvm/halt-polling.rst @@ -105,7 +105,7 @@ powerpc kvm-hv case. | | grow_halt_poll_ns() | | | | function. | | +-----------------------+---------------------------+-------------------------+ -|halt_poll_ns_shrink | The value by which the | 0 | +|halt_poll_ns_shrink | The value by which the | 2 | | | halt polling interval is | | | | divided in the | | | | shrink_halt_poll_ns() | | diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c index 486800a7024b..cfc474558660 100644 --- a/virt/kvm/kvm_main.c +++ b/virt/kvm/kvm_main.c @@ -91,8 +91,8 @@ unsigned int halt_poll_ns_grow_start = 10000; /* 10us */ module_param(halt_poll_ns_grow_start, uint, 0644); EXPORT_SYMBOL_GPL(halt_poll_ns_grow_start); -/* Default resets per-vcpu halt_poll_ns . */ -unsigned int halt_poll_ns_shrink; +/* Default halves per-vcpu halt_poll_ns. */ +unsigned int halt_poll_ns_shrink = 2; module_param(halt_poll_ns_shrink, uint, 0644); EXPORT_SYMBOL_GPL(halt_poll_ns_shrink); -- 2.17.1
