Red Hat Bugzilla – Bug 103189
SETSCHEDULER man page is unclear about FIFO scheduling
Last modified: 2007-04-18 12:57:10 EDT
From Bugzilla Helper: User-Agent: Mozilla/5.0 (X11; U; Linux i586; en-US; rv:1.5a) Gecko/20030801 Mozilla Firebird/0.6.1 Description of problem: The man page for sched_setscheduler is unclear as to how a running SCHED_FIFO'd process responds when a process which has also been SCHED_FIFO'd of the same priority comes along. The man page contains the following description: "When a SCHED_FIFO process becomes runnable, it will be inserted at the end of the list for its priority. A call to sched_setscheduler or sched_setparam will put the SCHED_FIFO (or SCHED_RR) process identified by pid at the start of the list if it was runnable. As a consequence, it may preempt the currently running process if it has the same priority." Later it goes on to say: "A SCHED_FIFO process runs until either it is blocked by an I/O request, it is preempted by a higher priority process, or it calls sched_yield." As such it is not wholely clear what happens if I set up one process with a scheduler of SCHED_FIFO and another process with the same scheduler priority also with SCHED_FIFO. Does the second preempt the first (as the first quote suggests) or does the second remained queued until the first blocks or yields (as the second quote suggests)? Does preemption only happen when the scheduler for the the second process is changed? Version-Release number of selected component (if applicable): man-pages-1.53-3 How reproducible: Always Steps to Reproduce: 1. man sched_setscheduler 2. Read the chapter titled SCHED_FIFO: First In-First Out scheduling 3. Try to work out whehther a a running SCHED_FIFO task will be preempted by another SCHED_FIFO task of the same priority and if so when it will happen. Actual Results: !cONFusioN Expected Results: Clarity. Additional info:
According to man-pages-1.66, the sentence in between your two sentences says "No other events will move a process scheduled ... in the wait list ... with equal static priority." and it says that POSIX 1003.1 says the process with go to the end of the list. So the answer is no, it will no be pre-empted if you later add a process of the same priority, which goes to the bottom of the list.