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

Peter Williams
2006-02-17T02:30:46Z

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@bigpond.com.au>
>>
>>Peter
>>--
>>Peter Williams pwil3058@bigpond.net.au
>>
>>"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;