1PIP STRESS(8) System Manager's Manual PIP STRESS(8)
2
6 pip_stress - Priority Inheritance with processes
7
9 pip_stress
10
13 This program demonstrates the technique of using priority inheritance
14 (PI) mutexes with processes instead of threads. The way to do this is
15 to obtain some shared memory - in this case with mmap that backs a
16 pthread_mutex_t since this will support PI. Pay particular attention
17 to how this is intialized to support processes. Function
18 init_shared_pthread_mutex() does this by setting the pthread_mutexattr
19 to PTHREAD_PROCESS_SHARED and the mutex protocol to
20 PTHREAD_PRIO_INHERIT. In this program we purposely try to invoke a
21 classic priority inversion. A low priority process grabs the mutex and
22 does some work. A high priority process comes a long and is blocked
23 since the mutex is taken. A medium priority process that doesn't
24 require the mutex then takes the processor. Because the processes are
25 restricted to one cpu, the low priority processes never makes any
26 progress because the medium priority process runs in an infinite loop.
27 This is a priority inversion because the medium priority process is
28 running at the expensive of the high priority process. However, since
29 we have used PRIO_INHERIT and are running on a machine that supports
30 preemption, the high priority process will lend it's priority to the
31 low priority process which will preempt the medium priority process.
32 The low priority process will then release the mutex which the high
33 priority process can obtain. When the high priority process gets to run
34 it kills the medium priority process. The state structure keeps track
35 of the progress. Although this program is set up to likely trigger an
36 inversion, there is no guarantee that scheduling will make that happen.
37 After the program completes it reports whether a priority inversion
38 occurred or not. In either case this program demonstrates how to use
39 priority inheritance mutexes with processes. In fact, you would be
40 better off to avoid scenarios in which a priority inversion occurs if
41 possible - this program tries to trigger them just to show that it
42 works. If you are having difficulty triggering an inversion, merely
43 increase the time that the low priority process sleeps while holding
44 the lock. (usleep); Also note that you have to run as a user with per‐
45 mission to change scheduling priorities.
46
48 pip_stress was written by John Kacur <jkacur at redhat.com>
49 This manual page was also written by John Kacur
51 September 17, 2018 PIP STRESS(8)