Peter Zijlstra wrote:
> On Mon, 2008-02-18 at 20:18 +0530, Balbir Singh wrote:
>
>>> Humm, the check that should have avoided that is:
>>>
>>> /*
>>> * Are we the only task in the tree?
>>> */
>>> if (unlikely(rq->load.weight == curr->se.load.weight))
>>> return;
>>>
>>>
>>> But I guess that overlooks rt tasks, they also increase the load.
>>> So I guess something like this ought to fix it..
>>>
>> Peter,
>>
>> I don't remember any real time tasks running on the system, so I would be
>> surprised if that is indeed the case.
>
> Various kthreads have rt prio. Notably the load_balancer_monitor().
>
OK, but does it belong to the cfs_rq?
>> Having said that, rightmost was indeed
>> NULL, so I need to figure out why it was. The other question is why would a
>> real
>> time task be found by sched_yield_fair?
>
> Because a rt task contributes weight and would make the test above fail
> because rt->load would be larger than expected.
>
I thought we were searching an RBtree for the fair group scheduler. If what you
say is indeed true, shouldn't we check if the task is an rt task in
sched_yield_fair() instead of the !rightmost check?
>>> diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
>>> index b9ade89..83eb30c 100644
>>> --- a/kernel/sched_fair.c
>>> +++ b/kernel/sched_fair.c
>>> @@ -998,7 +998,7 @@ static void yield_task_fair(struct rq *rq)
>>> /*
>>> * Already in the rightmost position?
>>> */
>>> - if (unlikely(rightmost->vruntime < se->vruntime))
>>> + if (unlikely(!rightmost || rightmost->vruntime < se->vruntime))
>>> return;
>>>
>>> /*
>>>
>
--
Warm Regards,
Balbir Singh
Linux Technology Center
IBM, ISTL
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