Documentation/scheduler/sched-rt-group.rst

2 Real-Time group scheduling
28    resolution, or the time it takes to handle the budget refresh itself.
33    are real-time processes).
42 Real-time scheduling is all about determinism, a group has to be able to rely on
43 the amount of bandwidth (eg. CPU time) being constant. In order to schedule
44 multiple groups of real-time tasks, each group must be assigned a fixed portion
45 of the CPU time available.  Without a minimum guarantee a real-time group can
52 CPU time is divided by means of specifying how much time can be spent running
53 in a given period. We allocate this "run time" for each real-time group which
54 the other real-time groups will not be permitted to use.
56 Any time not allocated to a real-time group will be used to run normal priority
57 tasks (SCHED_OTHER). Any allocated run time not used will also be picked up by
60 Let's consider an example: a frame fixed real-time renderer must deliver 25
63 time dedicated for the graphics. We can then give this group a run time of 0.8
66 This way the graphics group will have a 0.04s period with a 0.032s run time
68 needs only about 3% CPU time to do so, it can do with a 0.03 * 0.005s =
69 0.00015s. So this group can be scheduled with a period of 0.005s and a run time
72 The remaining CPU time will be used for user input and other tasks. Because
73 real-time tasks have explicitly allocated the CPU time they need to perform
93   A global limit on how much time real-time scheduling may use. This is always
94   less or equal to the period_us, as it denotes the time allocated from the
95   period_us for the real-time tasks. Even without CONFIG_RT_GROUP_SCHED enabled,
96   this will limit time reserved to real-time processes. With
98   real-time groups.
100   * Time is specified in us because the interface is s32. This gives an
104   * A run time of -1 specifies runtime == period, ie. no limit.
113 real-time tasks will not lock up the machine but leave a little time to recover
117 period from /proc/sys/kernel/sched_rt_period_us and a run time of 0. If you
121 Real-time group scheduling means you have to assign a portion of total CPU
122 bandwidth to the group before it will accept real-time tasks. Therefore you will
123 not be able to run real-time tasks as any user other than root until you have
124 done that, even if the user has the rights to run processes with real-time
135 to control the CPU time reserved for each control group.
178 get their allocated time.