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 <[email protected]>
Peter
--
Peter Williams [email protected]
"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 [email protected]
"Learning, n. The kind of ignorance distinguishing the studious."
-- Ambrose Bierce
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