Re: [PATCH] Fix smpnice high priority task hopping problem

[Peter Williams]

Siddha, Suresh B wrote:
> Andrew, Please don't apply this patch. This breaks the existing HT
> (and multi-core) scheduler optimizations.
>
> Peter, on a DP system with HT, if we have only two runnable processes
> and they end up running on the two threads of the same package, 
> with your patch, migration thread will never move one of those processes 
> to the idle package..

On a normal system, would either of them be moved anyway?

> To fix my reported problem, we need to make sure that find_busiest_group()
> doesn't find an imbalance..

I disagree.  If this causes a problem with your "optimizations" then I 
think that you need to fix the "optimizations".

There's a rational argument (IMHO) that this patch should be applied 
even in the absence of the smpnice patches as it prevents 
active_load_balance() doing unnecessary work.  If this isn't good for 
hypo threading then hypo threading is a special case and needs to handle 
it as such.

> thanks,
> suresh
>
> On Thu, Feb 16, 2006 at 11:39:34AM +1100, Peter Williams wrote:
>
> > Suresh B. Siddha has reported:
> >
> > "on a lightly loaded system, this will result in higher priority job 
> > hopping around from one processor to another processor.. This is because 
> > of the code in find_busiest_group() which assumes that SCHED_LOAD_SCALE 
> > represents a unit process load and with nice_to_bias calculations this 
> > is no longer true (in the presence of non nice-0 tasks)"
> >
> > Analysis of this problem as revealed that the smpnice code results in 
> > the weighted load being larger than 1 and this triggers the active load 
> > balancing code.  However, in active_load_balance(), the migration thread 
> > fails to take into account itself when deciding if there are any tasks 
> > to be migrated from its run queue. I.e. even if there is only one other 
> > task on the run queue other than itself it will still migrate that other 
> > task.
> >
> > The attached patch fixes that anomaly.
> >
> > Signed-off-by: Peter Williams <pwil3058@xxxxxxxxxxxxxx>
> >
> > Peter
> > --
> > Peter Williams                                   pwil3058@xxxxxxxxxxxxxx
> >
> > "Learning, n. The kind of ignorance distinguishing the studious."
> >  -- Ambrose Bierce
>
>
> > Index: MM-2.6.X/kernel/sched.c
> > ===================================================================
> > --- MM-2.6.X.orig/kernel/sched.c	2006-02-16 10:51:52.000000000 +1100
> > +++ MM-2.6.X/kernel/sched.c	2006-02-16 11:02:45.000000000 +1100
> > @@ -2406,7 +2406,7 @@ static void active_load_balance(runqueue
> > 	runqueue_t *target_rq;
> > 	int target_cpu = busiest_rq->push_cpu;
> >
> > -	if (busiest_rq->nr_running <= 1)
> > +	if (busiest_rq->nr_running <= 2)
> > 		/* no task to move */
> > 		return;


--
Peter Williams                                   pwil3058@xxxxxxxxxxxxxx

"Learning, n. The kind of ignorance distinguishing the studious."
 -- Ambrose Bierce
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