[PATCH v6 0/3] KVM: Dynamic Halt-Polling

From: Wanpeng Li
Date: Wed Sep 02 2015 - 03:29:55 EST

v5 -> v6:
* fix wait_ns and poll_ns

v4 -> v5:
* set base case 10us and max poll time 500us
* handle short/long halt, idea from David, many thanks David ;-)

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 always-poll since poll is still happened for idle
vCPUs which can waste cpu usage. This patchset add the ability to adjust
halt_poll_ns dynamically, to grow halt_poll_ns when shot halt is detected,
and to shrink halt_poll_ns when long halt is detected.

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

no-poll always-poll dynamic-poll
Idle (nohz) vCPU %c0 0.15% 0.3% 0.2%
Idle (250HZ) vCPU %c0 1.1% 4.6%~14% 1.2%
TCP_RR latency 34us 27us 26.7us

"Idle (X) vCPU %c0" is the percent of time the physical cpu spent in
c0 over 60 seconds (each vCPU is pinned to a pCPU). (nohz) means the
guest was tickless. (250HZ) means the guest was ticking at 250HZ.

The big win is with ticking operating systems. Running the linux guest
with nohz=off (and HZ=250), we save 3.4%~12.8% CPUs/second and get close
to no-polling overhead levels by using the dynamic-poll. The savings
should be even higher for higher frequency ticks.

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 | 70 ++++++++++++++++++++++++++++++++++++++++++----
3 files changed, 96 insertions(+), 5 deletions(-)


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