mirror of https://github.com/mkerrisk/man-pages
eaf4a2607b
The placement of a thread in the run queue for its new priority depends on the direction of movement in priority. (This appears to contradict POSIX, except in the case of pthread_setschedprio().) As reported by Andrea, and followed up by me: > I point out that the semantics of sched_setscheduler(2) for RT threads > indicated in sched(7) and, in particular, in > > "A call to sched_setscheduler(2), sched_setparam(2), or > sched_setattr(2) will put the SCHED_FIFO (or SCHED_RR) thread > identified by pid at the start of the list if it was runnable." > > does not "reflect" the current implementation of this syscall(s) that, in > turn; based on the source, I think a more appropriate description of this > semantics would be: > > "... the effect on its position in the thread list depends on the > direction of the modification, as follows: > > a. if the priority is raised, the thread becomes the tail of the > thread list. > b. if the priority is unchanged, the thread does not change position > in the thread list. > c. if the priority is lowered, the thread becomes the head of the > thread list." > > (copied from > http://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html#tag_15_08_04_01 > ). So, I did some testing, and can confirm that the above is the behavior on Linux for changes to scheduling priorities for RT processes. (My tests consisted of creating a multithreaded process where all threads are confined to the same CPU with taskset(), and each thread is in a CPU-bound loop. I then maipulated their priorities with chrt(1) and watched the CPU time being consumed with ps(1).) Back in SUSv2 there was this text: [[ 6. If a thread whose policy or priority has been modified is a running thread or is runnable, it then becomes the tail of the thread list for its new priority. ]] And certainly Linux used to behave this way. I remember testing it, and when one looks at the Linux 2.2 source code for example, one can see that there is a call to move_first_runqueue() in this case. At some point, things changed, and I have not investigated exactly where that change occurred (but I imagine it was quite a long time ago). Looking at SUSv4, let's expand the range of your quote, since point 7 is interesting. Here's text from Section 2.8.4 "Process Scheduling" in POSIX.1-2008/SUSv4 TC2: [[ 7. If a thread whose policy or priority has been modified other than by pthread_setschedprio() is a running thread or is runnable, it then becomes the tail of the thread list for its new priority. 8. If a thread whose priority has been modified by pthread_setschedprio() is a running thread or is runnable, the effect on its position in the thread list depends on the direction of the modification, as follows: a. If the priority is raised, the thread becomes the tail of the thread list. b. If the priority is unchanged, the thread does not change position in the thread list. c. If the priority is lowered, the thread becomes the head of the thread list. ]] (Note that the preceding points mention variously sched_setscheduler(), sched_setsparam(), and pthread_setschedprio(), so that the mention of just pthread_setschedprio() in points 7 and 8 is significant.) Now, since chrt(1) uses sched_setscheduler(), rather than pthread_setschedprio(), then arguably the Linux behavior is a violation of POSIX. (Indeed, buried in the man-pages source, I find that I many years ago wrote the comment: In 2.2.x and 2.4.x, the thread is placed at the front of the queue In 2.0.x, the Right Thing happened: the thread went to the back -- MTK But the Linux behavior seems reasonable to me and I'm inclined to just document it (see the patch below). Reported-by: Andrea Parri <parri.andrea@gmail.com> Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com> |
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README
This package contains Linux man pages for sections 1 through 8. Some more information is given in the 'man-pages-x.y.Announce' file. Installing and uninstalling =========================== "make install" will copy these man pages to /usr/share/man/man[1-8]. To install to a path different from /usr, use "make install prefix=/install/path". "make remove" or "make uninstall" will remove any man page in this distribution from its destination. Use with caution, and remember to use "prefix" if desired, as with the "install" target. "make" or "make all" will perform "make uninstall" followed by "make install". Man page overlap and duplication ================================ Note that sometimes these pages are duplicates of pages also distributed in other packages. This has been reported about: man page also found in ------------------------------------- resolver.3 bind-utils, bind9utils resolv.conf.5 " passwd.5 shadow, passwd mailaddr.7 ? Copyrights ========== See the 'man-pages-x.y.Announce' file. Homepage ======== For much more about the Linux man-pages project, see http://www.kernel.org/doc/man-pages/index.html.