Re: [RFC] sched: Limit idle_balance() when it is being used too frequently

[Jason Low <jason.low2@hp.com>]

On Wed, 2013-07-17 at 20:01 +0200, Peter Zijlstra wrote:
> On Wed, Jul 17, 2013 at 01:51:51PM -0400, Rik van Riel wrote:
> > On 07/17/2013 12:18 PM, Peter Zijlstra wrote:
> 
> > >So the way I see things is that the only way newidle balance can slow down
> > >things is if it runs when we could have ran something useful.
> > 
> > Due to contention on the runqueue locks of other CPUs,
> > newidle also has the potential to keep _others_ from
> > running something useful.
> 
> Right, although that should only happen when we do have an imbalance and want
> to go move something. Which in Jason's case is 'rare'. But yes, I suppose
> there's other scenarios where this is far more likely.
> 
> > Could we prevent that downside by measuring both the
> > time spent idle, and the time spent in idle balancing,
> > and making sure the idle balancing time never exceeds
> > more than N% of the idle time?
> 
> Sure:
> 
> idle_balance(u64 idle_duration)
> {
>   u64 cost = 0;
> 
>   for_each_domain(sd) {
>     if (cost + sd->cost > idle_duration/N)
>       break;
> 
>     ...
> 
>     sd->cost = (sd->cost + this_cost) / 2;
>     cost += this_cost;
>   }
> }
> 
> I would've initially suggested using something like N=2 since we're dealing
> with averages and half should ensure we don't run over except for the worst
> peaks. But we could easily use a bigger N.

Okay, I'll try this out. Thank you for your suggestions.

Jason.