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When level==0(low-performance mode): 1. prefer prev_cpu for rt tasks if prev cpu is fit. 2. make sure that it saves at least 6% of the energy when migrate tasks from little cpu to big cpu. When level==2(high-performance mode): 1. do not use EAS path. 2. select big cpu first when system is not overutilized. 3. do not trigger load_balance() when system is not overutilized. 4. prefer prev_cpu for rt tasks if prev cpu is fit. Test performance improvement for level==2: 1. CONFIG_ROCKCHIP_PERFORMANCE=n EMMC Random Write(4KB) 25.44MB/s Antutu: Total 581266 CPU 133023 GPU 234106 MEM 103602 UX 110535 2. CONFIG_ROCKCHIP_PERFORMANCE=y and level==2 EMMC Random Write(4KB) 44.19MB/s (73.7% improvement) Antutu: Total 600483 (3.3% improvement) CPU 134481 (1.1% improvement) GPU 234678 MEM 116551 (12.5% improvement) UX 114773 (3.8% improvement) Change-Id: I949ac229864eb12159b886b7769e0b489345bef4 Signed-off-by: Liang Chen <cl@rock-chips.com>