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

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

Peter Williams wrote:
> 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.

OK.  The good news is that (my testing shows that) the "sched: fix 
smpnice abnormal nice anomalies" fixes the imbalance problem and the 
consequent CPU hopping.

BUT I still think that this patch (modified if necessary to handle any 
HT special cases) should be applied.  On a normal system, it will (as 
I've already said) stop active_load_balance() from doing a lot of 
unnecessary work INCLUDING holding the run queue locks for TWO run 
queues for no good reason.

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

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