71 lines
3.1 KiB
Diff
71 lines
3.1 KiB
Diff
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From 2342d8be62a510569f8cbc9fe41574b6bc370073 Mon Sep 17 00:00:00 2001
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From: John Kacur <jkacur@redhat.com>
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Date: Mon, 17 Sep 2018 23:13:53 +0200
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Subject: [PATCH 3/5] rt-tests: pip_stress: Add an initial man page for
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pip_stress
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This adds a man page for pip_stress
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Signed-off-by: John Kacur <jkacur@redhat.com>
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---
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src/pi_tests/pip_stress.8 | 47 +++++++++++++++++++++++++++++++++++++++++++++++
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1 file changed, 47 insertions(+)
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create mode 100644 src/pi_tests/pip_stress.8
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diff --git a/src/pi_tests/pip_stress.8 b/src/pi_tests/pip_stress.8
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new file mode 100644
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index 000000000000..1808330b2e17
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--- /dev/null
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+++ b/src/pi_tests/pip_stress.8
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@@ -0,0 +1,47 @@
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+.\"
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+.TH PIP\ STRESS 8 "September 17, 2018"
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+.SH NAME
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+.B pip_stress \- Priority Inheritance with processes
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+.SH SYNOPSIS
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+.B pip_stress
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+
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+.SH DESCRIPTION
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+This program demonstrates the technique of using priority inheritance (PI)
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+mutexes with processes instead of threads.
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+The way to do this is to obtain some shared memory - in this case with
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+mmap that backs a pthread_mutex_t since this will support PI.
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+Pay particular attention to how this is intialized to support processes.
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+Function init_shared_pthread_mutex() does this by setting the
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+pthread_mutexattr to PTHREAD_PROCESS_SHARED and the mutex protocol to
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+PTHREAD_PRIO_INHERIT.
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+In this program we purposely try to invoke a classic priority inversion.
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+A low priority process grabs the mutex and does some work.
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+A high priority process comes a long and is blocked since the mutex is taken.
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+A medium priority process that doesn't require the mutex then takes the
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+processor. Because the processes are restricted to one cpu, the low priority
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+processes never makes any progress because the medium priority process
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+runs in an infinite loop. This is a priority inversion because the
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+medium priority process is running at the expensive of the high priority
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+process. However, since we have used PRIO_INHERIT and are running on a
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+machine that supports preemption, the high priority process will lend it's
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+priority to the low priority process which will preempt the medium priority
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+process. The low priority process will then release the mutex which the
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+high priority process can obtain. When the high priority process gets to run
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+it kills the medium priority process.
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+The state structure keeps track of the progress. Although this program
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+is set up to likely trigger an inversion, there is no guarantee that
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+scheduling will make that happen. After the program completes it reports
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+whether a priority inversion occurred or not. In either case this program
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+demonstrates how to use priority inheritance mutexes with processes.
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+In fact, you would be better off to avoid scenarios in which a priority
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+inversion occurs if possible - this program tries to trigger them just
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+to show that it works. If you are having difficulty triggering an inversion,
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+merely increase the time that the low priority process sleeps while
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+holding the lock. (usleep);
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+Also note that you have to run as a user with permission to change
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+scheduling priorities.
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+.BR
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+.SH AUTHOR
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+pip_stress was written by John Kacur <jkacur at redhat.com>
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+.PP
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+This manual page was also written by John Kacur
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--
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2.14.4
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