On 05/26/2014 01:19 PM, Vincent Guittot wrote:
> On 25 May 2014 12:33, Preeti U Murthy <pre...@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 <pre...@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) {
}
Regards
Preeti U Murthy
>
> Vincent
>
>>
>> Regards
>> Preeti U Murthy
>>
>
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
