Re: [PATCH] sched: make update_sd_pick_busiest return true on a busier sd

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Jul 22, 2014 at 02:45:59PM -0400, Rik van Riel wrote:
> Currently update_sd_pick_busiest only returns true when an sd
> is overloaded, or for SD_ASYM_PACKING when a domain is busier
> than average and a higher numbered domain than the target.
> 
> This breaks load balancing between domains that are not overloaded,
> in the !SD_ASYM_PACKING case. This patch makes update_sd_pick_busiest
> return true when the busiest sd yet is encountered.
> 
> On a 4 node system, this seems to result in the load balancer finally
> putting 1 thread of a 4 thread test run of "perf bench numa mem" on
> each node, where before the load was generally not spread across all
> nodes.

So for !ASYM the code is effectively:

	return sgs->avg_load > sds->busiest_stat.avg_load;

I'd like to at least add a clause that makes overloaded groups
prioritized over !overloaded groups.

Also, like we found earlier, calculate_imbalance() relies on the
sum_nr_running > group_capacity_factor thing, which you've just
'wrecked', so we'd need an update to that part too.

> Behaviour for SD_ASYM_PACKING does not seem to match the comment,
> in that groups with below average load average are ignored, but I
> have no hardware to test that so I have left the behaviour of that
> code unchanged.

Mikey, does that stuff work as expected?