linux-kernel - kernel bug, nice level should not affect SCHED

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Message-ID: <45EC7740.6040708@nortel.com>
Date:	Mon, 05 Mar 2007 14:02:08 -0600
From:	"Chris Friesen" <cfriesen@...tel.com>
To:	linux-kernel@...r.kernel.org
Subject: kernel bug, nice level should not affect SCHED_RR timeslice


Apparently the timeslice of the SCHED_RR process varies with nice level 
the same way that it does for SCHED_OTHER.  So while niceness doesn't 
affect the priority of a SCHED_RR task, it does impact how much cpu it gets.

SUSv3 indicates, "Any processes or threads using SCHED_FIFO or SCHED_RR 
shall be unaffected by a call to setpriority()."

The code in set_user_nice() has a comment that leads me to believe the 
current behaviour is accidental (although I think the "not" in the last 
line isn't meant to be there):

/*
  * The RT priorities are set via sched_setscheduler(), but we still
  * allow the 'normal' nice value to be set - but as expected
  * it wont have any effect on scheduling until the task is
  * not SCHED_NORMAL/SCHED_BATCH:
  */

It appears that the desired behaviour is to allow setting the nice level 
of a realtime task, but to not have it affect anything until (and 
unless) it drops that realtime status.  Is this correct?

Chris
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