16.3. big.LITTLE MP

For big.LITTLE MP on the Linux kernel the fundamental requirement is for the scheduler to decide when a software thread can run on a LITTLE core or a big core. The scheduler does this by comparing the tracked load of software threads against tunable load thresholds, an up migration threshold and a down migration threshold as shown in Figure 16.4.

Figure 16.4. Migration thresholds

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When the tracked load average of a thread currently allocated to a LITTLE core exceeds the up migration threshold, the thread is considered eligible for migration to a big core. Conversely, when the load average of a thread that is currently allocated to a big core drops below the down migration threshold, it is considered eligible for migration to a LITTLE core. In big.LITTLE MP, these basic rules govern task migration between big and LITTLE cores. Within the clusters, standard Linux scheduler load balancing applies. This tries to keep the load balanced across all the cores in one cluster.

The model is refined by adjusting the tracked load metric based on the current frequency of a core. A task that is running when the core is running at half speed, accrues tracked load at half the rate that it would if the core was running at full speed. This enables big.LITTLE MP and DVFS management to work together in harmony.

big.LITTLE MP uses several mechanisms to determine when to migrate a task between big and LITTLE cores:

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