Re: [RFCv5 PATCH 36/46] sched: Prevent unnecessary active balance of single task in sched group

From: Peter Zijlstra
Date: Sat Aug 15 2015 - 15:52:21 EST

On Tue, Jul 07, 2015 at 07:24:19PM +0100, Morten Rasmussen wrote:
> Scenarios with the busiest group having just one task and the local
> being idle on topologies with sched groups with different numbers of
> cpus manage to dodge all load-balance bailout conditions resulting the
> nr_balance_failed counter to be incremented. This eventually causes an
> pointless active migration of the task. This patch prevents this by not
> incrementing the counter when the busiest group only has one task.
> ASYM_PACKING migrations and migrations due to reduced capacity should
> still take place as these are explicitly captured by
> need_active_balance().
> A better solution would be to not attempt the load-balance in the first
> place, but that requires significant changes to the order of bailout
> conditions and statistics gathering.

*groan*, and this is of course triggered by your 2+3 core TC2 thingy.

Yes, asymmetric groups like that are a pain.
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