[PATCH v2 01/11] sched: fix imbalance flag reset

Preeti U Murthy preeti at linux.vnet.ibm.com
Mon May 26 02:16:14 PDT 2014

On 05/26/2014 01:19 PM, Vincent Guittot wrote:
> On 25 May 2014 12:33, Preeti U Murthy <preeti at linux.vnet.ibm.com> wrote:
>> Hi Vincent,
>> On 05/23/2014 09:22 PM, Vincent Guittot wrote:
>>> The imbalance flag can stay set whereas there is no imbalance.
>>> Let assume that we have 3 tasks that run on a dual cores /dual cluster system.
>>> We will have some idle load balance which are triggered during tick.
>>> Unfortunately, the tick is also used to queue background work so we can reach
>>> the situation where short work has been queued on a CPU which already runs a
>>> task. The load balance will detect this imbalance (2 tasks on 1 CPU and an idle
>>> CPU) and will try to pull the waiting task on the idle CPU. The waiting task is
>>> a worker thread that is pinned on a CPU so an imbalance due to pinned task is
>>> detected and the imbalance flag is set.
>>> Then, we will not be able to clear the flag because we have at most 1 task on
>>> each CPU but the imbalance flag will trig to useless active load balance
>>> between the idle CPU and the busy CPU.
>> Why do we do active balancing today when there is at-most 1 task on the
>> busiest cpu? Shouldn't we be skipping load balancing altogether? If we
>> do active balancing when the number of tasks = 1, it will lead to a ping
>> pong right?
> That's the purpose of the patch to prevent this useless active load
> balance. When the imbalance flag is set, an active load balance is
> triggered whatever the load balance is because of pinned tasks that
> prevents a balance state.

No I mean this:

sched:Do not continue load balancing when the busiest cpu has one
running task

From: Preeti U Murthy <preeti at linux.vnet.ibm.com>

 kernel/sched/fair.c |    2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index c9617b7..b175333 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6626,6 +6626,8 @@ more_balance:
 			goto out_balanced;
+	} else {
+		goto out;

 	if (!ld_moved) {


Preeti U Murthy
> Vincent
>> Regards
>> Preeti U Murthy

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