linux-kernel - Re: Regression with sched yield

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Date:	Mon, 18 Feb 2008 20:49:34 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	Ingo Molnar <mingo@...e.hu>,
	Dhaval Giani <dhaval@...ux.vnet.ibm.com>,
	Srivatsa Vaddagiri <vatsa@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>,
	linux kernel mailing list <linux-kernel@...r.kernel.org>
Subject: Re: Regression with sched yield - 2.6.25-rc2-mm1

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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