New: man sched_rr_get_interval says the interval depend on nice, but it's incorrect.

https://bugzilla.kernel.org/show_bug.cgi?id=27052

Summary: man sched_rr_get_interval says the interval depend on
nice, but it's incorrect.
Product: Documentation
Version: unspecified
Platform: All
OS/Version: Linux
Tree: Mainline
Status: NEW
Severity: normal
Priority: P1
Component: man-pages
AssignedTo: documentation_man-pages-ztI5WcYan/[email protected]
ReportedBy: [email protected]
Regression: No

man sched_rr_get_interval says SCHED_RR quantum depend on nice value.

----------------------------------------------------------------------------------
Linux Notes
POSIX does not specify any mechanism for controlling the size of
the
round-robin time quantum. However, Linux provides a (non-portable)
method
of doing this. The quantum can be controlled by adjusting the
process's
nice value (see setpriority(2)). Assigning a negative (i.e., high)
nice
value results in a longer quantum; assigning a positive (i.e., low)
nice
value results in a shorter quantum. The default quantum is 0.1
seconds;
the degree to which changing the nice value affects the quantum has
varied
somewhat across kernel versions.
----------------------------------------------------------------------------------

But current code doesn't depent on it. it always use 0.1 seconds. Hehe, The code comments is misleading too. It's not knob. ;)

----------------------------------------------------------------------------------
/*
* These are the 'tuning knobs' of the scheduler:
*
* default timeslice is 100 msecs (used only for SCHED_RR tasks).
* Timeslices get refilled after they expire.
*/
#define DEF_TIMESLICE (100 * HZ / 1000)

(snip)

static void task_tick_rt(struct rq *rq, struct task_struct *p, int queued)

{
        update_curr_rt(rq);

        watchdog(rq, p);

        /*
         * RR tasks need a special form of timeslice management.
         * FIFO tasks have no timeslices.
         */
        if (p->policy != SCHED_RR)
                return;

        if (--p->rt.time_slice)
                return;

        p->rt.time_slice = DEF_TIMESLICE;

        /*
         * Requeue to the end of queue if we are not the only element
         * on the queue:
         */
        if (p->rt.run_list.prev != p->rt.run_list.next) {
                requeue_task_rt(rq, p, 0);
                set_tsk_need_resched(p);
        }
}

(snip)

static unsigned int get_rr_interval_rt(struct rq *rq, struct task_struct *task)

{
        /*
         * Time slice is 0 for SCHED_FIFO tasks
         */
        if (task->policy == SCHED_RR)
                return DEF_TIMESLICE;
        else
                return 0;
}

----------------------------------------------------------------------------------

I think the man page only describe O(1) scheduler.

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https://bugzilla.kernel.org/show_bug.cgi?id=27052

Alan <alan-qBU/[email protected]> changed:

What |Removed |Added
----------------------------------------------------------------------------

             Status|NEW                         |RESOLVED
                 CC|                            |alan-qBU/[email protected]
         Resolution|                            |OBSOLETE

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