realtime:documentation:howto:applications:application_base
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realtime:documentation:howto:applications:application_base [2017/12/05 08:22] anna-maria adapt link to new page |
realtime:documentation:howto:applications:application_base [2024/05/31 15:54] (current) alison [Scheduling and priority] |
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| the pthread attributes and not to use the inherit scheduling of the | the pthread attributes and not to use the inherit scheduling of the | ||
| thread which created the real-time thread. | 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> | </wrap> | ||
| Line 39: | Line 43: | ||
| See [[realtime:documentation:howto:applications:memory#Stack Memory for RT threads | here]] | See [[realtime:documentation:howto:applications:memory#Stack Memory for RT threads | here]] | ||
| + | |||
| + | ==== 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''. | ||
| + | |||
| + | 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//. | ||
| + | |||
| + | 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 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). ... | ||
| + | |||
| + | |||
| + | **//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)! | ||
| + | |||
| + | And you're all set to run it as non-root now, a much more secure approach. | ||
| + | |||
| ===== Example ===== | ===== Example ===== | ||
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