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When a CPU is used to handle a lot of IRQs or some RT tasks, the remaining capacity for CFS tasks can be significantly reduced. Once we detect such situation by comparing cpu_capacity_orig and cpu_capacity, we trig an idle load balance to check if it's worth moving its tasks on an idle CPU. Once the idle load_balance has selected the busiest CPU, it will look for an active load balance for only two cases : - there is only 1 task on the busiest CPU. - we haven't been able to move a task of the busiest rq. A CPU with a reduced capacity is included in the 1st case, and it's worth to actively migrate its task if the idle CPU has got full capacity. This test has been added in need_active_balance. As a sidenote, this will note generate more spurious ilb because we already trig an ilb if there is more than 1 busy cpu. If this cpu is the only one that has a task, we will trig the ilb once for migrating the task. The nohz_kick_needed function has been cleaned up a bit while adding the new test env.src_cpu and env.src_rq must be set unconditionnally because they are used in need_active_balance which is called even if busiest->nr_running equals 1 Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
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