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

[Peter Williams <pwil3058@bigpond.net.au>]

Siddha, Suresh B wrote:
> On Fri, Feb 17, 2006 at 01:30:43PM +1100, Peter Williams wrote:
> 
>>On a normal system, would either of them be moved anyway?
> 
> 
> Possible. Because when the migration thread runs it moves the current running
> task out of the processor and the checks in can_migrate_task() like
> "sd->nr_balance_failed > sd->cache_nice_tries" can result in cache hot task
> move to the idle package.. This is a round about way and we should not depend
> on this behavior..

So why does it need to be retained?

> 
> 
>>>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.
> 
> 
> active load balance is designed only with HT optimizations in mind. And now
> multi-core optimizations also use this active load balance. No one else uses
> active load balance.

I can see nothing in the source code that will cause 
active_load_balance() to be only run on hypo threaded systems.  Could 
you please provide some pointers to the mechanism that does this.

Peter
-- 
Peter Williams                                   pwil3058@bigpond.net.au

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