zbus Priority Boost

Overview

This sample implements a simple application that illustrates the priority boost feature. The application implements the below figure scenario. When the priority boost feature is disabled, the execution sequence presents a priority inversion problem, which may not affect much of the developer’s application. Those who want to avoid priority inversions between message subscribers and plain subscribers should use the priority boost strategy.

ZBUS_CHAN_DEFINE(chan_a,
                 int,
                 NULL,
                 NULL,
                 ZBUS_OBSERVERS(l1, ms1, ms2, s1, l2),
                 0
);
ZBus priority boost scenario example.

Note

The developer must use the zbus_obs_attach_to_thread() function to ensure a proper priority boost execution.

Building and Running

The figure below illustrates the execution of the cited scenario but with the priority boost disabled.

ZBus example scenario for priority boost disabled.

It can be built and executed on QEMU as follows:

# From the root of the zephyr repository
west build -b qemu_x86 samples/subsys/zbus/priority_boost -- -DCONFIG_ZBUS_PRIORITY_BOOST=n
west build -t run

Sample Output

I: --------------
I: 0 -> T1: prio before 5
I: 0 ---> L1: T1 prio 5
I: 0 -> MS1:  T1 prio 5
I: 0 -> MS2:  T1 prio 5
I: 0 ---> L2: T1 prio 5
I: 0 -> T1: prio after 5
I: N -> S1:  T1 prio 5
I: 0 -> S1:  T1 prio 5
I: --------------
I: 1 -> T1: prio before 5
I: 1 ---> L1: T1 prio 5
I: 1 -> MS1:  T1 prio 5
I: 1 -> MS2:  T1 prio 5
I: 1 ---> L2: T1 prio 5
I: 1 -> T1: prio after 5
I: N -> S1:  T1 prio 5
I: 1 -> S1:  T1 prio 5
I: --------------
I: 2 -> T1: prio before 5
I: 2 ---> L1: T1 prio 5
I: 2 -> MS1:  T1 prio 5
I: 2 ---> L2: T1 prio 5
I: 2 -> T1: prio after 5
I: 2 -> MS2:  T1 prio 5
I: --------------
I: 3 -> T1: prio before 5
I: 3 ---> L1: T1 prio 5
I: 3 -> MS1:  T1 prio 5
I: 3 ---> L2: T1 prio 5
I: 3 -> T1: prio after 5
I: 3 -> MS2:  T1 prio 5
I: --------------
I: 4 -> T1: prio before 5
I: 4 ---> L1: T1 prio 5
I: 4 ---> L2: T1 prio 5
I: 4 -> T1: prio after 5
I: 4 -> MS2:  T1 prio 5
I: --------------
I: 5 -> T1: prio before 5
I: 5 ---> L1: T1 prio 5
I: 5 ---> L2: T1 prio 5
I: 5 -> T1: prio after 5
I: 5 -> MS2:  T1 prio 5
I: --------------
I: 6 -> T1: prio before 5
I: 6 ---> L1: T1 prio 5
I: 6 -> MS1:  T1 prio 5
I: 6 -> MS2:  T1 prio 5
I: 6 ---> L2: T1 prio 5
I: 6 -> T1: prio after 5
I: N -> S1:  T1 prio 5
I: 6 -> S1:  T1 prio 5
I: --------------
<continues>

Exit QEMU by pressing CTRL+A x.

The figure below illustrates the execution of the same scenario but with the priority boost enabled. The developer must enable the priority boost and properly attach all the observers to their threads.

ZBus example scenario for priority boost enabled.

To execute the sample with priority boost feature enabled, run the following command:

# From the root of the zephyr repository
west build -b qemu_x86 samples/subsys/zbus/priority_boost -- -DCONFIG_ZBUS_PRIORITY_BOOST=y
west build -t run

Sample Output

I: --------------
I: 0 -> T1: prio before 5
I: 0 ---> L1: T1 prio 1
I: 0 ---> L2: T1 prio 1
I: N -> S1:  T1 prio 5
I: 0 -> S1:  T1 prio 5
I: 0 -> MS1:  T1 prio 5
I: 0 -> MS2:  T1 prio 5
I: 0 -> T1: prio after 5
I: --------------
I: 1 -> T1: prio before 5
I: 1 ---> L1: T1 prio 1
I: 1 ---> L2: T1 prio 1
I: N -> S1:  T1 prio 5
I: 1 -> S1:  T1 prio 5
I: 1 -> MS1:  T1 prio 5
I: 1 -> MS2:  T1 prio 5
I: 1 -> T1: prio after 5
I: --------------
I: 2 -> T1: prio before 5
I: 2 ---> L1: T1 prio 2
I: 2 ---> L2: T1 prio 2
I: 2 -> MS1:  T1 prio 5
I: 2 -> MS2:  T1 prio 5
I: 2 -> T1: prio after 5
I: --------------
I: 3 -> T1: prio before 5
I: 3 ---> L1: T1 prio 2
I: 3 ---> L2: T1 prio 2
I: 3 -> MS1:  T1 prio 5
I: 3 -> MS2:  T1 prio 5
I: 3 -> T1: prio after 5
I: --------------
I: 4 -> T1: prio before 5
I: 4 ---> L1: T1 prio 3
I: 4 ---> L2: T1 prio 3
I: 4 -> MS2:  T1 prio 5
I: 4 -> T1: prio after 5
I: --------------
I: 5 -> T1: prio before 5
I: 5 ---> L1: T1 prio 3
I: 5 ---> L2: T1 prio 3
I: 5 -> MS2:  T1 prio 5
I: 5 -> T1: prio after 5
I: --------------
I: 6 -> T1: prio before 5
I: 6 ---> L1: T1 prio 1
I: 6 ---> L2: T1 prio 1
I: N -> S1:  T1 prio 5
I: 6 -> S1:  T1 prio 5
I: 6 -> MS1:  T1 prio 5
I: 6 -> MS2:  T1 prio 5
I: 6 -> T1: prio after 5
I: --------------
<continues>

Exit QEMU by pressing CTRL+A x.

See also

Zbus APIs