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Documentation/scheduler/sched-deadline.txt: Improve and clarify AC bits

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Admission control is of key importance for SCHED_DEADLINE, since
it guarantees system schedulability (or tells us something about
the degree of guarantees we can provide to the user).

This patch improves and clarifies bits and pieces regarding AC,
both for UP and SMP systems.

Signed-off-by: Luca Abeni <luca.abeni@unitn.it>
Signed-off-by: Juri Lelli <juri.lelli@arm.com>
Reviewed-by: Henrik Austad <henrik@austad.us>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Dario Faggioli <raistlin@linux.it>
Cc: Juri Lelli <juri.lelli@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/1410256636-26171-4-git-send-email-juri.lelli@arm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Luca Abeni 2014-09-09 10:57:14 +01:00 committed by Ingo Molnar
parent 0d9ba8b03c
commit b56bfc6cd1
1 changed files with 80 additions and 15 deletions
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95
Documentation/scheduler/sched-deadline.txt
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@ -38,16 +38,17 @@ CONTENTS
==================
SCHED_DEADLINE uses three parameters, named "runtime", "period", and
"deadline" to schedule tasks. A SCHED_DEADLINE task is guaranteed to receive
"deadline", to schedule tasks. A SCHED_DEADLINE task should receive
"runtime" microseconds of execution time every "period" microseconds, and
these "runtime" microseconds are available within "deadline" microseconds
from the beginning of the period. In order to implement this behaviour,
every time the task wakes up, the scheduler computes a "scheduling deadline"
consistent with the guarantee (using the CBS[2,3] algorithm). Tasks are then
scheduled using EDF[1] on these scheduling deadlines (the task with the
earliest scheduling deadline is selected for execution). Notice that this
guaranteed is respected if a proper "admission control" strategy (see Section
"4. Bandwidth management") is used.
earliest scheduling deadline is selected for execution). Notice that the
task actually receives "runtime" time units within "deadline" if a proper
"admission control" strategy (see Section "4. Bandwidth management") is used
(clearly, if the system is overloaded this guarantee cannot be respected).
Summing up, the CBS[2,3] algorithms assigns scheduling deadlines to tasks so
that each task runs for at most its runtime every period, avoiding any
@ -134,6 +135,50 @@ CONTENTS
A real-time task can be periodic with period P if r_{j+1} = r_j + P, or
sporadic with minimum inter-arrival time P is r_{j+1} >= r_j + P. Finally,
d_j = r_j + D, where D is the task's relative deadline.
The utilisation of a real-time task is defined as the ratio between its
WCET and its period (or minimum inter-arrival time), and represents
the fraction of CPU time needed to execute the task.
If the total utilisation sum_i(WCET_i/P_i) is larger than M (with M equal
to the number of CPUs), then the scheduler is unable to respect all the
deadlines.
Note that total utilisation is defined as the sum of the utilisations
WCET_i/P_i over all the real-time tasks in the system. When considering
multiple real-time tasks, the parameters of the i-th task are indicated
with the "_i" suffix.
Moreover, if the total utilisation is larger than M, then we risk starving
non- real-time tasks by real-time tasks.
If, instead, the total utilisation is smaller than M, then non real-time
tasks will not be starved and the system might be able to respect all the
deadlines.
As a matter of fact, in this case it is possible to provide an upper bound
for tardiness (defined as the maximum between 0 and the difference
between the finishing time of a job and its absolute deadline).
More precisely, it can be proven that using a global EDF scheduler the
maximum tardiness of each task is smaller or equal than
((M − 1) · WCET_max − WCET_min)/(M − (M − 2) · U_max) + WCET_max
where WCET_max = max_i{WCET_i} is the maximum WCET, WCET_min=min_i{WCET_i}
is the minimum WCET, and U_max = max_i{WCET_i/P_i} is the maximum utilisation.
If M=1 (uniprocessor system), or in case of partitioned scheduling (each
real-time task is statically assigned to one and only one CPU), it is
possible to formally check if all the deadlines are respected.
If D_i = P_i for all tasks, then EDF is able to respect all the deadlines
of all the tasks executing on a CPU if and only if the total utilisation
of the tasks running on such a CPU is smaller or equal than 1.
If D_i != P_i for some task, then it is possible to define the density of
a task as C_i/min{D_i,T_i}, and EDF is able to respect all the deadlines
of all the tasks running on a CPU if the sum sum_i C_i/min{D_i,T_i} of the
densities of the tasks running on such a CPU is smaller or equal than 1
(notice that this condition is only sufficient, and not necessary).
On multiprocessor systems with global EDF scheduling (non partitioned
systems), a sufficient test for schedulability can not be based on the
utilisations (it can be shown that task sets with utilisations slightly
larger than 1 can miss deadlines regardless of the number of CPUs M).
However, as previously stated, enforcing that the total utilisation is smaller
than M is enough to guarantee that non real-time tasks are not starved and
that the tardiness of real-time tasks has an upper bound.
SCHED_DEADLINE can be used to schedule real-time tasks guaranteeing that
the jobs' deadlines of a task are respected. In order to do this, a task
@ -163,14 +208,22 @@ CONTENTS
4. Bandwidth management
=======================
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.
As previously mentioned, in order for -deadline scheduling to be
effective and useful (that is, to be able to provide "runtime" time units
within "deadline"), it is important to have some method to keep the allocation
of the available fractions of CPU time to the various tasks under control.
This is usually called "admission control" and if it is not performed, then
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
As already stated in Section 3, a necessary condition to be respected to
correctly schedule a set of real-time tasks is that the total utilisation
is smaller than M. When talking about -deadline tasks, this requires that
the sum of the ratio between runtime and period for all tasks is smaller
than M. Notice that the ratio runtime/period is equivalent to the utilisation
of a "traditional" real-time task, and is also often referred to as
"bandwidth".
The interface used to control the 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).
@ -182,9 +235,13 @@ CONTENTS
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 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).
desired bandwidth. In other words, this means that interface parameters are
only used at admission control time (i.e., when the user calls
sched_setattr()). Scheduling is then performed considering actual tasks'
parameters, so that CPU bandwidth is allocated to SCHED_DEADLINE tasks
respecting their needs in terms of granularity. 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
------------------------
@ -232,8 +289,16 @@ CONTENTS
950000. With rt_period equal to 1000000, by default, it means that -deadline
tasks can use at most 95%, multiplied by the number of CPUs that compose the
root_domain, for each root_domain.
This means that non -deadline tasks will receive at least 5% of the CPU time,
and that -deadline tasks will receive their runtime with a guaranteed
worst-case delay respect to the "deadline" parameter. If "deadline" = "period"
and the cpuset mechanism is used to implement partitioned scheduling (see
Section 5), then this simple setting of the bandwidth management is able to
deterministically guarantee that -deadline tasks will receive their runtime
in a period.
A -deadline task cannot fork.
Finally, notice that in order not to jeopardize the admission control a
-deadline task cannot fork.
5. Tasks CPU affinity
=====================