diff options
| -rw-r--r-- | Documentation/scheduler/sched-deadline.txt | 51 |
1 files changed, 25 insertions, 26 deletions
diff --git a/Documentation/scheduler/sched-deadline.txt b/Documentation/scheduler/sched-deadline.txt index a029891a8228..f75d8327b914 100644 --- a/Documentation/scheduler/sched-deadline.txt +++ b/Documentation/scheduler/sched-deadline.txt @@ -165,39 +165,38 @@ CONTENTS In order for the -deadline scheduling to be effective and useful, it is important to have some method to keep the allocation of the available CPU - bandwidth to the tasks under control. - This is usually called "admission control" and if it is not performed at all, - no guarantee can be given on the actual scheduling of the -deadline tasks. - - Since when RT-throttling has been introduced each task group has a bandwidth - associated, calculated as a certain amount of runtime over a period. - Moreover, to make it possible to manipulate such bandwidth, readable/writable - controls have been added to both procfs (for system wide settings) and cgroupfs - (for per-group settings). - Therefore, the same interface is being used for controlling the bandwidth - distrubution to -deadline tasks. - - However, more discussion is needed in order to figure out how we want to manage - SCHED_DEADLINE bandwidth at the task group level. Therefore, SCHED_DEADLINE - uses (for now) a less sophisticated, but actually very sensible, mechanism to - ensure that a certain utilization cap is not overcome per each root_domain. - - Another main difference between deadline bandwidth management and RT-throttling + bandwidth to the tasks under control. This is usually called "admission + control" and if it is not performed at all, no guarantee can be given on + the actual scheduling of the -deadline tasks. + + The interface used to control the fraction of CPU bandwidth that can be + allocated to -deadline tasks is similar to the one already used for -rt + tasks with real-time group scheduling (a.k.a. RT-throttling - see + Documentation/scheduler/sched-rt-group.txt), and is based on readable/ + writable control files located in procfs (for system wide settings). + Notice that per-group settings (controlled through cgroupfs) are still not + defined for -deadline tasks, because more discussion is needed in order to + figure out how we want to manage SCHED_DEADLINE bandwidth at the task group + level. + + A main difference between deadline bandwidth management and RT-throttling is that -deadline tasks have bandwidth on their own (while -rt ones don't!), - and thus we don't need an higher level throttling mechanism to enforce the - desired bandwidth. + and thus we don't need a higher level throttling mechanism to enforce the + desired bandwidth. Therefore, using this simple interface we can put a cap + on total utilization of -deadline tasks (i.e., \Sum (runtime_i / period_i) < + global_dl_utilization_cap). 4.1 System wide settings ------------------------ The system wide settings are configured under the /proc virtual file system. - For now the -rt knobs are used for dl admission control and the -deadline - runtime is accounted against the -rt runtime. We realise that this isn't - entirely desirable; however, it is better to have a small interface for now, - and be able to change it easily later. The ideal situation (see 5.) is to run - -rt tasks from a -deadline server; in which case the -rt bandwidth is a direct - subset of dl_bw. + For now the -rt knobs are used for -deadline admission control and the + -deadline runtime is accounted against the -rt runtime. We realise that this + isn't entirely desirable; however, it is better to have a small interface for + now, and be able to change it easily later. The ideal situation (see 5.) is to + run -rt tasks from a -deadline server; in which case the -rt bandwidth is a + direct subset of dl_bw. This means that, for a root_domain comprising M CPUs, -deadline tasks can be created while the sum of their bandwidths stays below: |