Embedded Software for SoC - Grupo de Mecatrônica EESC/USP

20 Chapter 2
Table 2-2.
RTOS functions timing.
RTOS functions
Measured time
value scratchpad
Measured time
value w/ cache
Measured time
value w/o cache
ActivateTask pre
A act
B act
C act
Task pre
TerminateTask pre
A term
B term
C term
TerminateTask post
that with proper RTOS characterization‚ it is possible to obtain tight conservative
bounds on RTOS overhead during task response time analysis.
Columns two and three in Table 2-2 show that the I-cache improved the
performance of some of the RTOS functions. This will also be considered in
the following section.
5.3. Single and networked ECU analysis
Single ECU analysis determines response times‚ and consequently schedulability
for all tasks running on the ECU. It builds upon single-process timing
(Section 5.1) and RTOS timing results (Section 5.2). On top of that‚ it considers
task-activation patterns and the scheduling strategy.
Our example system consists of three periodically activated tasks without
data-dependencies. The tasks are scheduled by a fixed-priority scheduler and
each task’s deadline is equal to its period. Such a system can be analyzed
using the static-priority preemptive analysis which was developed by Liu and
Layland [10] and extended in [16] to account for exclusive-resource access
arbitration. Additionally‚ we have to consider the RTOS overhead as explained
in the previous section. A snapshot of the possible behavior of our system is
shown in Figure 2-3.
During analysis‚ we can account for the high priority level of the X act and
Activate Task pre functions by treating them as independent periodic tasks
at a very high priority level. The X term function corresponding to the current
task will not influence the task response time. However‚ if a higher priority
task interrupts a running task then the preemption time includes also the
execution time of the X term function that corresponds to the preempting
task. Task pre is added to the process core execution time‚ and Terminate
Task pre and Terminate Task post to the execution time of the X term
function. We extend the analysis of [10] to capture this behavior and obtain
the worst case task response time.