Re: [PATCH] sched/fair: check for idle core

[Mel Gorman]

On Wed, Oct 21, 2020 at 03:24:48PM +0200, Julia Lawall wrote:
> > I worry it's overkill because prev is always used if it is idle even
> > if it is on a node remote to the waker. It cuts off the option of a
> > wakee moving to a CPU local to the waker which is not equivalent to the
> > original behaviour.
> 
> But it is equal to the original behavior in the idle prev case if you go
> back to the runnable load average days...
> 

It is similar but it misses the sync treatment and sd->imbalance_pct part of
wake_affine_weight which has unpredictable consequences. The data
available is only on the fully utilised case.

> The problem seems impossible to solve, because there is no way to know by
> looking only at prev and this whether the thread would prefer to stay
> where it was or go to the waker.
> 

Yes, this is definitely true. Looking at prev_cpu and this_cpu is a
crude approximation and the path is heavily limited in terms of how
clever it can be.

-- 
Mel Gorman
SUSE Labs