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

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Message-ID: <BLU437-SMTP37F331E727F970F8BE6182806F0@phx.gbl>
Date:	Thu, 27 Aug 2015 17:52:16 +0800
From:	Wanpeng Li <wanpeng.li@...mail.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
CC:	David Matlack <dmatlack@...gle.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, Wanpeng Li <wanpeng.li@...mail.com>
Subject: [PATCH v4 0/3] KVM: Dynamic Halt-Polling

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(-)
-- 
1.9.1

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