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

Peter Williams
2006-02-17T02:51:49Z

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.