Messages in this thread | | | Date | Wed, 17 Jul 2013 20:01:56 +0200 | From | Peter Zijlstra <> | Subject | Re: [RFC] sched: Limit idle_balance() when it is being used too frequently |
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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.
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