svn commit: r197652 - stable/7/sys/kern

[Zachary Loafman]

Author: zml
Date: Wed Sep 30 19:40:51 2009
New Revision: 197652
URL: http://svn.freebsd.org/changeset/base/197652

Log:
  sched_ule in stable/7 has a bug (introduced in r180607) where a thread
  that is running often will appear to not be running much at all.
  
  sched_ule has a much less accurate mechanism for determining how much
  various threads are running.  Every tick, hardclock_cpu() calls
  sched_tick(), and the currently running thread gets it's ts_ticks
  incremented.  Whenever an event of interest happens to a thread, the
  ts_ticks value may be decayed; it's supposed to be a rough running
  average of the last 10 seconds.  So there's a ts_ltick which is the last
  tick we looked at decaying ts_ticks.
  
  The increment in sched_tick() was slightly buggy on SMP, because a
  thread could get incremented on two different CPUs in the rare case
  where it was swapped from one which had run sched_tick() this tick to
  one which hadn't.  The fix that was used relied on ts_ltick and only
  incremented ts_ticks if ts_ltick was not from the current tick.
  This is buggy, because any time the thread began running on a CPU in the
  current tick, we would have set ts_ltick to ticks, so if it was still
  running at sched_tick() we wouldn't increment.
  
  A system with a single process that hogs the CPU and is otherwise idle,
  therefore, would look like all threads were at 0%. The threads not
  running are really at 0%, and the hog is not getting its ts_ticks
  incremented since it went through some other runq stats that set
  ts_ltick.  On a 2-way SMP the thread used to get shuffled regularly
  between CPUs (I think fallout from this bug), so it would appear a
  little over 50% busy.
  
  The fix is to use a separate variable to record when the last
  sched_tick() increment happened.
  
  Submitted by:       Matthew Fleming (matthew.fleming at isilon.com)
  Reviewed by:        zml, dfr
  Approved by:        dfr (mentor)

Modified:
  stable/7/sys/kern/sched_ule.c

Modified: stable/7/sys/kern/sched_ule.c
==============================================================================
--- stable/7/sys/kern/sched_ule.c	Wed Sep 30 19:19:53 2009	(r197651)
+++ stable/7/sys/kern/sched_ule.c	Wed Sep 30 19:40:51 2009	(r197652)
@@ -101,6 +101,7 @@ struct td_sched {	
 	u_int		ts_runtime;	/* Number of ticks we were running */
 	/* The following variables are only used for pctcpu calculation */
 	int		ts_ltick;	/* Last tick that we were running on */
+	int		ts_incrtick;	/* Last tick that we incremented on */
 	int		ts_ftick;	/* First tick that we were running on */
 	int		ts_ticks;	/* Tick count */
 #ifdef SMP
@@ -2075,6 +2076,7 @@ sched_fork_thread(struct thread *td, str
 	 */
 	ts2->ts_ticks = ts->ts_ticks;
 	ts2->ts_ltick = ts->ts_ltick;
+	ts2->ts_incrtick = ts->ts_incrtick;
 	ts2->ts_ftick = ts->ts_ftick;
 	child->td_user_pri = td->td_user_pri;
 	child->td_base_user_pri = td->td_base_user_pri;
@@ -2266,10 +2268,11 @@ sched_tick(void)
 	 * Ticks is updated asynchronously on a single cpu.  Check here to
 	 * avoid incrementing ts_ticks multiple times in a single tick.
 	 */
-	if (ts->ts_ltick == ticks)
+	if (ts->ts_incrtick == ticks)
 		return;
 	/* Adjust ticks for pctcpu */
 	ts->ts_ticks += 1 << SCHED_TICK_SHIFT;
+	ts->ts_incrtick = ticks;
 	ts->ts_ltick = ticks;
 	/*
 	 * Update if we've exceeded our desired tick threshhold by over one