Re: [RFC PATCH 1/1] fair.c: Add/Export find_idlest_perfer_cpu API

On Mon, 2012-08-20 at 14:00 +0200, Peter Zijlstra wrote:

On Fri, 2012-08-17 at 12:46 -0700, Shirley Ma wrote:

quoted

Add/Export a new API for per-cpu thread model networking device

driver

quoted

to choose a preferred idlest cpu within allowed cpumask.

The receiving CPUs of a networking device are not under cgroup

controls.

quoted

Normally the receiving work will be scheduled on the cpu on which

the

quoted

interrupts are received. When such a networking device uses per-cpu
thread model, the cpu which is chose to process the packets might

not be

quoted

part of cgroup cpusets without using such an API here. 

On NUMA system, by using the preferred cpumask from the same NUMA

node

quoted

would help to reduce expensive cross memory access to/from the other
NUMA node.

KVM per-cpu vhost will be the first one to use this API. Any other
device driver which uses per-cpu thread model and has cgroup cpuset
control will use this API later.

How often will this be called and how do you obtain the cpumasks
provided to the function? 

It depends. It might be called pretty often if the user keeps changing
cgroups control cpuset. It might be less called if the cgroups control
cpuset is stable, and the host scheduler always schedules the work on
the same NUMA node.

The preferred cpumasks are obtained from local numa node. The allowed
cpumasks are obtained from caller's task allowed cpumasks (cgroups
control cpuset).

Thanks
Shirley