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sched/fair: Relax constraint on task's load during load balance
Some UCs like 9 always running tasks on 8 CPUs can't be balanced and the load balancer currently migrates the waiting task between the CPUs in an almost random manner. The success of a rq pulling a task depends of the value of nr_balance_failed of its domains and its ability to be faster than others to detach it. This behavior results in an unfair distribution of the running time between tasks because some CPUs will run most of the time, if not always, the same task whereas others will share their time between several tasks. Instead of using nr_balance_failed as a boolean to relax the condition for detaching task, the LB will use nr_balanced_failed to relax the threshold between the tasks'load and the imbalance. This mecanism prevents the same rq or domain to always win the load balance fight. Signed-off-by: Vincent Guittot <[email protected]> Signed-off-by: Peter Zijlstra (Intel) <[email protected]> Reviewed-by: Phil Auld <[email protected]> Link: https://lkml.kernel.org/r/[email protected]
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