K 10
svn:author
V 3
zml
K 8
svn:date
V 27
2009-09-30T19:40:51.118006Z
K 7
svn:log
V 1767
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)

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