[Xen-devel] [RFC 0/6] XEN scheduling hardening

[Andrii Anisov <andrii.anisov@xxxxxxxxx>]

From: Andrii Anisov <andrii_anisov@xxxxxxxx>

This is the very RFC series, which is aimed to address some of VCPU time
accounting problems which affect scheduling fairness and accuracy. Please
note that this is done for ARM64 yet.

One of the scheduling problems is a misleading CPU idle time concept. Now
for the CPU idle time, it is taken an idle vcpu run time. But idle vcpu run
time includes IRQ processing, softirqs processing, tasklets processing, etc.
Those tasks are not actual idle and they accounting may mislead CPU freq
governors who rely on the CPU idle time. In this series, it is suggested to
take the time of the actual CPU low power mode as the idle time. 

The other problem is that pure hypervisor tasks execution time is charged from
the guest vcpu budget. For example, IRQ and softirq processing time are charged
from the current vcpu budget, which is likely the guest vcpu. This is quite
unfair and may break scheduling reliability. It is proposed to charge guest
vcpus for the guest actual run time and time to serve guest's hypercalls and
access to emulated iomem. All the rest is calculated as the hypervisor run time
(IRQ and softirq processing, branch prediction hardening, etc.)

While the series is the early RFC, several points are still untouched:
 - Now the time elapsed from the last rescheduling is not fully charged from
   the current vcpu budget. Are there any changes needed in the existing
   scheduling algorithms?
 - How to avoid the absolute top priority of tasklets (what is obeyed by all
   schedulers so far). Should idle vcpu be scheduled as the normal guest vcpus
   (through queues, priorities, etc)?
 - Idle vcpu naming is quite misleading. It is a kind of system (hypervisor)
   task which is responsible for some hypervisor work. Should it be
   renamed/reconsidered?

Andrii Anisov (5):
  schedule: account true system idle time
  sysctl: extend XEN_SYSCTL_getcpuinfo interface
  xentop: show CPU load information
  arm64: call enter_hypervisor_head only when it is needed
  schedule: account all the hypervisor time to the idle vcpu

Julien Grall (1):
  xen/arm: Re-enable interrupt later in the trap path

 tools/xenstat/libxenstat/src/xenstat.c      |  38 +++++++++
 tools/xenstat/libxenstat/src/xenstat.h      |   9 ++
 tools/xenstat/libxenstat/src/xenstat_priv.h |   3 +
 tools/xenstat/xentop/xentop.c               |  30 +++++++
 xen/arch/arm/arm64/entry.S                  |  17 ++--
 xen/arch/arm/domain.c                       |  24 ++++++
 xen/arch/arm/traps.c                        | 128 +++++++++++++++++++---------
 xen/common/sched_credit.c                   |   2 +-
 xen/common/sched_credit2.c                  |   4 +-
 xen/common/sched_rt.c                       |   2 +-
 xen/common/schedule.c                       |  98 ++++++++++++++++++---
 xen/common/sysctl.c                         |   2 +
 xen/include/public/sysctl.h                 |   2 +
 xen/include/xen/sched.h                     |   7 ++
 14 files changed, 303 insertions(+), 63 deletions(-)

-- 
2.7.4


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