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Diffstat (limited to 'Documentation/scheduler/sched-rt-group.txt')
| -rw-r--r-- | Documentation/scheduler/sched-rt-group.txt | 52 |
1 files changed, 29 insertions, 23 deletions
diff --git a/Documentation/scheduler/sched-rt-group.txt b/Documentation/scheduler/sched-rt-group.txt index 3ef339f491e..71b54d54998 100644 --- a/Documentation/scheduler/sched-rt-group.txt +++ b/Documentation/scheduler/sched-rt-group.txt @@ -4,6 +4,7 @@ CONTENTS ======== +0. WARNING 1. Overview 1.1 The problem 1.2 The solution @@ -14,6 +15,23 @@ CONTENTS 3. Future plans +0. WARNING +========== + + Fiddling with these settings can result in an unstable system, the knobs are + root only and assumes root knows what he is doing. + +Most notable: + + * very small values in sched_rt_period_us can result in an unstable + system when the period is smaller than either the available hrtimer + resolution, or the time it takes to handle the budget refresh itself. + + * very small values in sched_rt_runtime_us can result in an unstable + system when the runtime is so small the system has difficulty making + forward progress (NOTE: the migration thread and kstopmachine both + are real-time processes). + 1. Overview =========== @@ -55,7 +73,7 @@ The remaining CPU time will be used for user input and other tasks. Because realtime tasks have explicitly allocated the CPU time they need to perform their tasks, buffer underruns in the graphics or audio can be eliminated. -NOTE: the above example is not fully implemented as of yet (2.6.25). We still +NOTE: the above example is not fully implemented yet. We still lack an EDF scheduler to make non-uniform periods usable. @@ -108,28 +126,17 @@ priority! 2.3 Basis for grouping tasks ---------------------------- -There are two compile-time settings for allocating CPU bandwidth. These are -configured using the "Basis for grouping tasks" multiple choice menu under -General setup > Group CPU Scheduler: - -a. CONFIG_USER_SCHED (aka "Basis for grouping tasks" = "user id") - -This lets you use the virtual files under -"/sys/kernel/uids/<uid>/cpu_rt_runtime_us" to control he CPU time reserved for -each user . - -The other option is: - -.o CONFIG_CGROUP_SCHED (aka "Basis for grouping tasks" = "Control groups") +Enabling CONFIG_RT_GROUP_SCHED lets you explicitly allocate real +CPU bandwidth to task groups. -This uses the /cgroup virtual file system and "/cgroup/<cgroup>/cpu.rt_runtime_us" -to control the CPU time reserved for each control group instead. +This uses the cgroup virtual file system and "<cgroup>/cpu.rt_runtime_us" +to control the CPU time reserved for each control group. For more information on working with control groups, you should read -Documentation/cgroups.txt as well. +Documentation/cgroups/cgroups.txt as well. -Group settings are checked against the following limits in order to keep the configuration -schedulable: +Group settings are checked against the following limits in order to keep the +configuration schedulable: \Sum_{i} runtime_{i} / global_period <= global_runtime / global_period @@ -142,8 +149,7 @@ For now, this can be simplified to just the following (but see Future plans): =============== There is work in progress to make the scheduling period for each group -("/sys/kernel/uids/<uid>/cpu_rt_period_us" or -"/cgroup/<cgroup>/cpu.rt_period_us" respectively) configurable as well. +("<cgroup>/cpu.rt_period_us") configurable as well. The constraint on the period is that a subgroup must have a smaller or equal period to its parent. But realistically its not very useful _yet_ @@ -169,9 +175,9 @@ get their allocated time. Implementing SCHED_EDF might take a while to complete. Priority Inheritance is the biggest challenge as the current linux PI infrastructure is geared towards -the limited static priority levels 0-139. With deadline scheduling you need to +the limited static priority levels 0-99. With deadline scheduling you need to do deadline inheritance (since priority is inversely proportional to the -deadline delta (deadline - now). +deadline delta (deadline - now)). This means the whole PI machinery will have to be reworked - and that is one of the most complex pieces of code we have. |