1PIP STRESS(8) System Manager's Manual PIP STRESS(8)
2# SPDX-License-Identifier: GPL-2.0-or-later
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8 pip_stress - Priority Inheritance with processes
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11 pip_stress
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15 This program demonstrates the technique of using priority inheritance
16 (PI) mutexes with processes instead of threads. The way to do this is
17 to obtain some shared memory - in this case with mmap that backs a
18 pthread_mutex_t since this will support PI. Pay particular attention
19 to how this is intialized to support processes. Function
20 init_shared_pthread_mutex() does this by setting the pthread_mutexattr
21 to PTHREAD_PROCESS_SHARED and the mutex protocol to PTHREAD_PRIO_IN‐
22 HERIT. In this program we purposely try to invoke a classic priority
23 inversion. A low priority process grabs the mutex and does some work.
24 A high priority process comes a long and is blocked since the mutex is
25 taken. A medium priority process that doesn't require the mutex then
26 takes the processor. Because the processes are restricted to one cpu,
27 the low priority processes never makes any progress because the medium
28 priority process runs in an infinite loop. This is a priority inversion
29 because the medium priority process is running at the expensive of the
30 high priority process. However, since we have used PRIO_INHERIT and are
31 running on a machine that supports preemption, the high priority
32 process will lend it's priority to the low priority process which will
33 preempt the medium priority process. The low priority process will then
34 release the mutex which the high priority process can obtain. When the
35 high priority process gets to run it kills the medium priority process.
36 The state structure keeps track of the progress. Although this program
37 is set up to likely trigger an inversion, there is no guarantee that
38 scheduling will make that happen. After the program completes it re‐
39 ports whether a priority inversion occurred or not. In either case this
40 program demonstrates how to use priority inheritance mutexes with pro‐
41 cesses. In fact, you would be better off to avoid scenarios in which a
42 priority inversion occurs if possible - this program tries to trigger
43 them just to show that it works. If you are having difficulty trigger‐
44 ing an inversion, merely increase the time that the low priority
45 process sleeps while holding the lock. (usleep); Also note that you
46 have to run as a user with permission to change scheduling priorities.
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50 pip_stress was written by John Kacur <jkacur at redhat.com>
51 This manual page was also written by John Kacur
53 September 17, 2018 PIP STRESS(8)