Re: [PATCH, 2.6.9] improved load_balance() tolerance for pinned tasks

["John Hawkes" <hawkes@sgi.com>]

From: "Nick Piggin" <nickpiggin@yahoo.com.au>
> > From: "John Hawkes" <hawkes@google.engr.sgi.com>
> > Actually, there is another related problem that arises in
> > active_load_balance() with a runqueue that holds hundreds of pinned
processes.
> > I'm seeing a migration_thread perpetually consuming 70% of its CPU.
>
> That's what I was worried about, but in your most recent
> patch you just sent, the all_pinned path should skip over
> the active load balance completely... basically it shouldn't
> be running at all, and if it is then it is a bug I think?

To reiterate:  this is probably reproducible on smaller SMP systems, too.
Just do a 'runon' (using sys_sched_setaffinity) of ~200 (or more) small
computebound processes on a single CPU.

My patch -- that has load_balance() skip over (busiest->active_balance = 1)
trigger that starts up active_load_balance() -- does seem to reduce the
frequency of bursts of long-running activity of the migration thread, but
those burst of activity are still there, with migration_thread consuming
75-95% of its CPU for several seconds (as observed by 'top').  I have not yet
determined what's happening.  It might be an artifact of how long it takes to
do those 'runon' startups of the computebound processes.

John Hawkes