By default Linux is tuned for performance. Which means that RT tasks always run at the highest frequency and most capable (highest capacity) CPU (in heterogeneous systems).
Uclamp achieves this by setting the requested uclamp.min of all RT tasks to 1024 by default, which effectively boosts the tasks to run at the highest frequency and biases them to run on the biggest CPU.
This knob allows admins to change the default behavior when uclamp is being used. In battery powered devices particularly, running at the maximum capacity and frequency will increase energy consumption and shorten the battery life.
This knob is only effective for RT tasks which the user hasn’t modified their requested uclamp.min value via sched_setattr() syscall.
This knob will not escape the range constraint imposed by sched_util_clamp_min defined above.
For example if
sched_util_clamp_min_rt_default = 800
sched_util_clamp_min = 600
Then the boost will be clamped to 600 because 800 is outside of the permissible range of [0:600]. This could happen for instance if a powersave mode will restrict all boosts temporarily by modifying sched_util_clamp_min. As soon as this restriction is lifted, the requested sched_util_clamp_min_rt_default will take effect.