Re: [PATCH v4 0/3] KVM: Dynamic Halt-Polling

From: Wanpeng Li
Date: Sat Aug 29 2015 - 18:22:06 EST

Hi Peter,
On 8/30/15 5:18 AM, Peter Kieser wrote:
Hi Wanpeng,

Do I need to set any module parameters to use your patch, or should halt_poll_ns automatically tune with just your patch series applied?

You don't need any module parameters.

Wanpeng Li


On 2015-08-27 2:47 AM, Wanpeng Li wrote:
v3 -> v4:
* bring back grow vcpu->halt_poll_ns when interrupt arrives and shrinks
when idle VCPU is detected

v2 -> v3:
* grow/shrink vcpu->halt_poll_ns by *halt_poll_ns_grow or /halt_poll_ns_shrink
* drop the macros and hard coding the numbers in the param definitions
* update the comments "5-7 us"
* remove halt_poll_ns_max and use halt_poll_ns as the max halt_poll_ns time,
vcpu->halt_poll_ns start at zero
* drop the wrappers
* move the grow/shrink logic before "out:" w/ "if (waited)"

v1 -> v2:
* change kvm_vcpu_block to read halt_poll_ns from the vcpu instead of
the module parameter
* use the shrink/grow matrix which is suggested by David
* set halt_poll_ns_max to 2ms

There is a downside of halt_poll_ns since poll is still happen for idle
VCPU which can waste cpu usage. This patchset add the ability to adjust
halt_poll_ns dynamically, grows halt_poll_ns if an interrupt arrives and
shrinks halt_poll_ns when idle VCPU is detected.

There are two new kernel parameters for changing the halt_poll_ns:
halt_poll_ns_grow and halt_poll_ns_shrink.

Test w/ high cpu overcommit ratio, pin vCPUs, and the halt_poll_ns of
halt-poll is the default 500000ns, the max halt_poll_ns of dynamic
halt-poll is 2ms. Then watch the %C0 in the dump of Powertop tool.
The test method is almost from David.

| | | |
| w/o halt-poll | w/ halt-poll | dynamic halt-poll |
| | | |
| ~0.9% | ~1.8% | ~1.2% |
The always halt-poll will increase ~0.9% cpu usage for idle vCPUs and the
dynamic halt-poll drop it to ~0.3% which means that reduce the 67% overhead
introduced by always halt-poll.

Wanpeng Li (3):
KVM: make halt_poll_ns per-VCPU
KVM: dynamic halt_poll_ns adjustment
KVM: trace kvm_halt_poll_ns grow/shrink

include/linux/kvm_host.h | 1 +
include/trace/events/kvm.h | 30 ++++++++++++++++++++++++++++
virt/kvm/kvm_main.c | 50 +++++++++++++++++++++++++++++++++++++++++++---
3 files changed, 78 insertions(+), 3 deletions(-)

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