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

From: Peter Williams
Date: Thu Feb 16 2006 - 21:34:06 EST


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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