Re: [PATCH] Avoid moving tasks when a schedule can be made.

[Steven Rostedt <rostedt@goodmis.org>]

On Wed, 2006-02-01 at 14:08 +0100, Ingo Molnar wrote:
> * Steven Rostedt <rostedt@goodmis.org> wrote:
> 
> [pls. use -p when generating patches]
> 
> > @@ -1983,6 +1983,10 @@
> >  
> >  	curr = curr->prev;
> >  
> > +	/* bail if someone else woke up */
> > +	if (need_resched())
> > +		goto out;
> > +
> >  	if (!can_migrate_task(tmp, busiest, this_cpu, sd, idle, &pinned)) {
> >  		if (curr != head)
> >  			goto skip_queue;
> 
> even putting the problems of this approach aside (is it right to abort 
> the act of load-balancing - which is a periodic activity that wont be 
> restarted after this - so we lose real work), i think this will not 
> solve the latency. Imagine a hardirq hitting the CPU that is executing 
> move_tasks() above. We might not service that hardirq for up to 1.5 
> msecs ...
> 
> i think the right approach would be to split up this work into smaller 
> chunks. Or rather, lets first see how this can happen: why is 
> can_migrate() false for so many tasks? Are they all cpu-hot? If yes, 
> shouldnt we simply skip only up to a limit of tasks in this case - it's 
> not like we want to spend 1.5 msecs searching for a cache-cold task 
> which might give us a 50 usecs advantage over cache-hot tasks ...
> 

OK, agreed.

Just to clear things up.  I looked further into what was causing this,
and the can_migrate was indeed true, and it just happened that we got a
large imbalance, and it pulled a few hundred tasks.  So, my earlier
analysis was incorrect about the can_migrate being false.

-- Steve