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--- realtime:documentation:howto:applications:application_base [2023/08/29 07:25]
kaiwan
+++ realtime:documentation:howto:applications:application_base [2024/05/31 15:54] (current)
alison [Scheduling and priority]
@@ Line 30: / Line 30: @@
 the pthread attributes and not to use the inherit scheduling of the
 thread which created the real-time thread.
+</wrap>
+  - **Problems with pthread condition variables** \\ <wrap>
+Multithreaded applications which rely on glibc's libpthread are prone to unexpected latency delays since its condition variable implementation does not honor priority inheritance ([[https://sourceware.org/bugzilla/show_bug.cgi?id=11588|bugzilla]]). Unfortunately glibc's DNS resolver and asynchronous I/O implementations depend in turn on these condition variables.
+[[https://github.com/dvhart/librtpi|librtpi]] is an alternative LGPL-licensed pthread implementation which supports priority inheritance, and whose API is as close to glibc's as possible.  The alternative [[https://www.musl-libc.org/|MUSL libc]] has a pthread condition variable implementation similar to glibc's.
 </wrap>
@@ Line 42: / Line 46: @@
 ==== Capabilities: running the app with RT priority as a non-root user ====
-Several of the Pthread APIs, like mlockall(), pthread_attr_setschedpolicy(), by default and convention require root in order to successfully get their work done. Thus, RT apps - which need to set an RT sched policy and priority -  are often run via sudo.
+Several of the Pthread APIs, like ''mlockall()'', ''pthread_attr_setschedpolicy()'', by default and convention require root in order to successfully get their work done. Thus, RT apps - which need to set an RT sched policy and priority -  are often run via ''sudo''.
-There's a far better approach to this; sudo gives the process root capabilities. This interests hackers :-).
+There's a far better approach to this; ''sudo'' gives the process root capabilities. This interests hackers :-).
-Instead, you should leverage the powerful POSIX Capabilities model! This way, the process (and threads) get _only_ the capabilities they require and nothing more. This follows the infosec best practice, the 'principle of least privilege'.
+Instead, you should leverage the powerful POSIX **Capabilities** model! This way, the process (and threads) get _only_ the capabilities they require and nothing more. This follows the infosec best practice, the //principle of least privilege//.
-Apps start out with no capabilities by default; also note that capabilities are a per-thread resource (essentially translating to bitmasks with the task structure,which is per-thread of course). Among the various capability bits, the man page on capabilities(7) shows that **CAP_SYS_NICE** is the appropriate capability to use in this circumstance; a snippet from the man page reveals this:
+Apps start out with no capabilities by default; also note that capabilities are a per-thread resource (essentially translating to bitmasks with the task structure,which is per-thread of course). Among the various capability bits, the man page on ''capabilities(7)'' shows that **''CAP_SYS_NICE''** is the appropriate capability to use in this circumstance; a snippet from the ''capabilities(7)'' man page reveals this:
-...
+...
 **CAP_SYS_NICE**
   * Lower the process nice value (nice(2), setpriority(2)) and change the nice value for arbitrary processes;
-  * set  real-time  scheduling  policies  for  calling process, and set scheduling policies and priorities for arbitrary processes (sched_setscheduler(2), sched_setparam(2), sched_setattr(2));
+  * **set  real-time  scheduling  policies  for  calling process, and set scheduling policies and priorities for arbitrary processes (sched_setscheduler(2), sched_setparam(2), sched_setattr(2));**
   * set CPU affinity for arbitrary processes (sched_setaffinity(2));
   * set I/O scheduling class and priority for arbitrary processes (ioprio_set(2));
   * apply migrate_pages(2) to arbitrary processes and allow processes to be migrated to arbitrary nodes;
   * apply move_pages(2) to arbitrary processes;
-  * use the MPOL_MF_MOVE_ALL flag with mbind(2) and move_pages(2).
+  * use the MPOL_MF_MOVE_ALL flag with mbind(2) and move_pages(2). ...
-...
+**//Ok, great, but how exactly is this capability bit to be set on the app?//
+**
+  - One approach is to do so programatically, via the ''capget()/capset()'' system calls. (Note that's it's generally easier to use the libcap library wrappers, ''cap_[g|s]et_proc(3)'': [[https://man7.org/linux/man-pages/man3/cap_get_proc.3.html]]. This man page even provides a small example of doing so).
+  - Another easy way is to leverage systemd and run your app as a service; in the service unit, specify the capability (see the man page on systemd.exec(5); [[https://www.freedesktop.org/software/systemd/man/systemd.exec.html#Capabilities]].
+  - Perhaps the easiest way: via the ''setcap(8)'' utility (it's man page: [[https://man7.org/linux/man-pages/man8/setcap.8.html]]). The setcap/getcap are typically part of the libcap package. For example:
+    ''sudo setcap CAP_SYS_NICE+eip <your-app-binary-executable>''
+You could put this line in the app Makefile (or equivalent).
+(The ''getcap(8)'' utility can be used to verify that the 'dumb-capability' binary now has the ''CAP_SYS_NICE'' bit set)!
-//Ok, great, but how exactly is this capability bit to be set on the app?//
+And you're all set to run it as non-root now, a much more secure approach.
-  - One approach is to do so programatically, via the capget()/capset() system calls. (Note that's it's generally easier to use the libpcap library wrappers, cap_[g|s]et_proc(3): [[https://man7.org/linux/man-pages/man3/cap_get_proc.3.html]]. This man page even provides a small example of doing so).
-  - Another is to use systemd and run your app as a service; in the service unit, specify the capability (see the man page on systemd.exec(5); [[https://www.freedesktop.org/software/systemd/man/systemd.exec.html#Capabilities]].
-  - Perhaps the easiest way: via the setcap(8) utility (it's man page: [[https://man7.org/linux/man-pages/man8/setcap.8.html]]).
 ===== Example =====

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