Documentation/scheduler/sched-rt-group.rst

2 Real-Time group scheduling
12      2.1 System-wide settings
21  Fiddling with these settings can result in an unstable system, the knobs are
27    system when the period is smaller than either the available hrtimer
31    system when the runtime is so small the system has difficulty making
33    are real-time processes).
40 ---------------
42 Real-time scheduling is all about determinism, a group has to be able to rely on
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
50 ----------------
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
60 Let's consider an example: a frame fixed real-time renderer must deliver 25
73 real-time tasks have explicitly allocated the CPU time they need to perform
77 lack an EDF scheduler to make non-uniform periods usable.
84 2.1 System wide settings
85 ------------------------
87 The system wide settings are configured under the /proc virtual file system:
93   A global limit on how much time real-time scheduling may use. This is always
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.
103   * sched_rt_runtime_us takes values from -1 to sched_rt_period_us.
104   * A run time of -1 specifies runtime == period, ie. no limit.
108 ---------------------
112 SCHED_OTHER (non-RT tasks). These defaults were chosen so that a run-away
113 real-time tasks will not lock up the machine but leave a little time to recover
114 it.  By setting runtime to -1 you'd get the old behaviour back.
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
129 ----------------------------
134 This uses the cgroup virtual file system and "<cgroup>/cpu.rt_runtime_us"
138 Documentation/admin-guide/cgroup-v1/cgroups.rst as well.
165 	- this runs for 0.05s once every 0.1s
169 	- this runs for 0.025s twice every 0.1s (or once every 0.05 sec).
182 the limited static priority levels 0-99. With deadline scheduling you need to
184 deadline delta (deadline - now)).
186 This means the whole PI machinery will have to be reworked - and that is one of