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sched/dl/Documentation: Add some references

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Add a description of the Dhall's effect, some discussion about
schedulability tests for global EDF, and references to real-time
literature.

Signed-off-by: Luca Abeni <luca.abeni@unitn.it>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: henrik@austad.us
Cc: juri.lelli@gmail.com
Cc: raistlin@linux.it
Link: http://lkml.kernel.org/r/1431954032-16473-8-git-send-email-luca.abeni@unitn.it
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Luca Abeni 2015-05-18 15:00:30 +02:00 committed by Ingo Molnar
parent e0deda8142
commit 134136c4b7
1 changed files with 67 additions and 6 deletions
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73
Documentation/scheduler/sched-deadline.txt
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@ -163,7 +163,8 @@ CONTENTS
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{WCET_i} is the maximum WCET, WCET_min=min{WCET_i}
is the minimum WCET, and U_max = max{WCET_i/P_i} is the maximum utilization.
is the minimum WCET, and U_max = max{WCET_i/P_i} is the maximum
utilization[12].
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
@ -205,11 +206,48 @@ CONTENTS
On multiprocessor systems with global EDF scheduling (non partitioned
systems), a sufficient test for schedulability can not be based on the
utilizations (it can be shown that task sets with utilizations slightly
larger than 1 can miss deadlines regardless of the number of CPUs M).
However, as previously stated, enforcing that the total utilization 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.
utilizations or densities: it can be shown that even if D_i = P_i task
sets with utilizations slightly larger than 1 can miss deadlines regardless
of the number of CPUs.
Consider a set {Task_1,...Task_{M+1}} of M+1 tasks on a system with M
CPUs, with the first task Task_1=(P,P,P) having period, relative deadline
and WCET equal to P. The remaining M tasks Task_i=(e,P-1,P-1) have an
arbitrarily small worst case execution time (indicated as "e" here) and a
period smaller than the one of the first task. Hence, if all the tasks
activate at the same time t, global EDF schedules these M tasks first
(because their absolute deadlines are equal to t + P - 1, hence they are
smaller than the absolute deadline of Task_1, which is t + P). As a
result, Task_1 can be scheduled only at time t + e, and will finish at
time t + e + P, after its absolute deadline. The total utilization of the
task set is U = M · e / (P - 1) + P / P = M · e / (P - 1) + 1, and for small
values of e this can become very close to 1. This is known as "Dhall's
effect"[7]. Note: the example in the original paper by Dhall has been
slightly simplified here (for example, Dhall more correctly computed
lim_{e->0}U).
More complex schedulability tests for global EDF have been developed in
real-time literature[8,9], but they are not based on a simple comparison
between total utilization (or density) and a fixed constant. If all tasks
have D_i = P_i, a sufficient schedulability condition can be expressed in
a simple way:
sum(WCET_i / P_i) <= M - (M - 1) · U_max
where U_max = max{WCET_i / P_i}[10]. Notice that for U_max = 1,
M - (M - 1) · U_max becomes M - M + 1 = 1 and this schedulability condition
just confirms the Dhall's effect. A more complete survey of the literature
about schedulability tests for multi-processor real-time scheduling can be
found in [11].
As seen, enforcing that the total utilization is smaller than M does not
guarantee that global EDF schedules the tasks without missing any deadline
(in other words, global EDF is not an optimal scheduling algorithm). However,
a total utilization 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[12] (as previously noted). Different bounds on the maximum tardiness
experienced by real-time tasks have been developed in various papers[13,14],
but the theoretical result that is important for SCHED_DEADLINE is that if
the total utilization is smaller or equal than M then the response times of
the tasks are limited.
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
@ -245,6 +283,29 @@ CONTENTS
Concerning the Preemptive Scheduling of Periodic Real-Time tasks on
One Processor. Real-Time Systems Journal, vol. 4, no. 2, pp 301-324,
1990.
7 - S. J. Dhall and C. L. Liu. On a real-time scheduling problem. Operations
research, vol. 26, no. 1, pp 127-140, 1978.
8 - T. Baker. Multiprocessor EDF and Deadline Monotonic Schedulability
Analysis. Proceedings of the 24th IEEE Real-Time Systems Symposium, 2003.
9 - T. Baker. An Analysis of EDF Schedulability on a Multiprocessor.
IEEE Transactions on Parallel and Distributed Systems, vol. 16, no. 8,
pp 760-768, 2005.
10 - J. Goossens, S. Funk and S. Baruah, Priority-Driven Scheduling of
Periodic Task Systems on Multiprocessors. Real-Time Systems Journal,
vol. 25, no. 2–3, pp. 187–205, 2003.
11 - R. Davis and A. Burns. A Survey of Hard Real-Time Scheduling for
Multiprocessor Systems. ACM Computing Surveys, vol. 43, no. 4, 2011.
http://www-users.cs.york.ac.uk/~robdavis/papers/MPSurveyv5.0.pdf
12 - U. C. Devi and J. H. Anderson. Tardiness Bounds under Global EDF
Scheduling on a Multiprocessor. Real-Time Systems Journal, vol. 32,
no. 2, pp 133-189, 2008.
13 - P. Valente and G. Lipari. An Upper Bound to the Lateness of Soft
Real-Time Tasks Scheduled by EDF on Multiprocessors. Proceedings of
the 26th IEEE Real-Time Systems Symposium, 2005.
14 - J. Erickson, U. Devi and S. Baruah. Improved tardiness bounds for
Global EDF. Proceedings of the 22nd Euromicro Conference on
Real-Time Systems, 2010.
4. Bandwidth management
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