Re: [PATCH] sched: Introduce scaled capacity awareness in enqueue

[Rohit Jain]

On 05/30/2017 05:28 AM, Peter Zijlstra wrote:
> On Mon, May 22, 2017 at 01:48:14PM -0700, Rohit Jain wrote:
> > The patch introduces capacity awarness in scheduler (CAS) which avoids
> > CPUs which might have their capacities reduced (due to IRQ/RT activity)
> > when trying to schedule threads (on the push side) in the system. This
> > awareness has been added into the fair scheduling class.
> >
> > It does so by, using the following algorithm:
> > --------------------------------------------------------------------------
> > 1) As in rt_avg the scaled capacities are already calculated.
> >
> > 2) This scaled capacity is normalized and mapped into buckets.
> Why?

This is done to deterministically define the CPUs which are low on
capacities. By mapping it into buckets it becomes easier to do a
percentile calculation.

> > 3) Any CPU which lies below the 80th percentile in terms of percentage
> > capacity available is considered as a low capacity CPU.
> Random number; can we do better? What does existing code do?

Existing code checks to see if the waker is running on a CPU with better
capacity than the 'previous cpu'. If that is the case the waker cpu is
provided as 'target' to select_idle_sibling. (When I say capacity this
is not the scaled one because of IRQ, RT, etc.)

However, in case this 'target' is not an idle CPU, the rest of
select_idle_sibling would ignore the capacities altogether.