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I want to create two kernels for the Raspberry Pi 3. One running all the processes with only the SCHED_FIFO policy, the second running the processes with only the SCHED_RR policy.

Can somebody help me with the function's modifications I must do in the source code of Raspbian Kernel 4.9? I am totally new to this field.

According to my survey, the changes have to be done in the sched_fork and the _sched_setscheduler functions in kernel/sched/core.c but I am confused about which thing to touch here.

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A possible solution is outlined here: https://aelseb.wordpress.com/2016/01/06/change-linux-cpu-default-scheduler/

sched_fork:
+     /* Lorenzo Nava: force policy to RR */
+     if (p->policy == SCHED_NORMAL) {
+         p->prio = current->normal_prio - NICE_WIDTH -
+                 PRIO_TO_NICE(current->static_prio);
+         p->normal_prio = p->prio;
+         p->rt_priority = p->prio;
+         p->policy = SCHED_RR;
+         p->static_prio = NICE_TO_PRIO(0);
+     }

_sched_setscheduler:
+     /* Lorenzo Nava: force policy of process to RR */
+     if (attr.sched_policy == SCHED_NORMAL) {
+         attr.sched_priority = param->sched_priority -
+                 NICE_WIDTH - attr.sched_nice;
+         attr.sched_policy = SCHED_RR;
+     }
+

It still applies to the Kernel 4.12 which I am currently using.

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