[RFC PATCH 0/2] Adjust CFS loadbalance to adapt QEMU CPU topology.

[Kenan.Liu]

From: "Kenan.Liu" <Kenan.Liu@xxxxxxxxxxxxxxxxx>

Multithreading workloads in VM with Qemu may encounter an unexpected
phenomenon: one hyperthread of a physical core is busy while its sibling
is idle. Such as:

%Cpu0  : 19.8 us,  3.8 sy,  0.0 ni, 76.5 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
%Cpu1  :  6.4 us,  1.0 sy,  0.0 ni, 92.6 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
%Cpu2  : 15.8 us,  4.5 sy,  0.0 ni, 79.7 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
%Cpu3  :  4.7 us,  0.7 sy,  0.0 ni, 94.6 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
%Cpu4  : 15.5 us,  4.5 sy,  0.0 ni, 80.1 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
%Cpu5  :  4.7 us,  0.7 sy,  0.0 ni, 94.6 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
%Cpu6  : 13.4 us,  3.4 sy,  0.0 ni, 83.2 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
%Cpu7  :  2.7 us,  0.3 sy,  0.0 ni, 97.0 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
%Cpu8  : 16.1 us,  4.8 sy,  0.0 ni, 79.1 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
%Cpu9  :  2.0 us,  0.3 sy,  0.0 ni, 97.7 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
%Cpu10 : 17.5 us,  5.2 sy,  0.0 ni, 77.3 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
%Cpu11 :  0.0 us,  0.0 sy,  0.0 ni,100.0 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
%Cpu12 : 17.6 us,  4.5 sy,  0.0 ni, 77.9 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
%Cpu13 :  0.3 us,  0.0 sy,  0.0 ni, 99.7 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
%Cpu14 : 16.1 us,  4.1 sy,  0.0 ni, 79.8 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
%Cpu15 :  0.0 us,  0.0 sy,  0.0 ni,100.0 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st

The main reason is that hyperthread index is consecutive in qemu native x86 CPU
model which is different from the physical topology. As the current kernel scheduler
implementation, hyperthread with an even ID number will be picked up in a much
higher probability during load-balancing and load-deploying.

This RFC targets to solve the problem by adjusting CFS loabalance policy:
1. Explore CPU topology and adjust CFS loadbalance policy when we found machine
with qemu native CPU topology.
2. Export a procfs to control the traverse length when select idle cpu.

Kenan.Liu (2):
  sched/fair: Adjust CFS loadbalance for machine with qemu native CPU
    topology.
  sched/fair: Export a param to control the traverse len when select
    idle cpu.

 kernel/sched/fair.c | 103 ++++++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 99 insertions(+), 4 deletions(-)

-- 
1.8.3.1