linux-kernel - Re: [PATCH] Fix migration of SCHED

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Date:	Thu, 15 Oct 2015 17:40:19 +0100
From:	Juri Lelli <juri.lelli@....com>
To:	Luca Abeni <luca.abeni@...tn.it>, linux-kernel@...r.kernel.org
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Wanpeng Li <wanpeng.li@...ux.intel.com>
Subject: Re: [PATCH] Fix migration of SCHED_DEADLINE tasks

Hi Luca,

On 15/10/15 12:09, Luca Abeni wrote:
> Commit 9d5142624256 ("sched/deadline: Reduce rq lock contention by
> eliminating locking of non-feasible target") broke select_task_rq_dl()
> and find_lock_later_rq(), because it introduced a comparison between
> the local task's deadline and dl.earliest_dl.curr of the remote queue.
> However, if the remote runqueue does not contain any SCHED_DEADLINE
> task its earliest_dl.curr is 0 (always smaller than the deadline of
> the local task) and the remote runqueue is not selected for pushing.
> As a result, if an application creates multiple SCHED_DEADLINE threads,
> they will never be pushed to runqueues that do not already contain
> SCHED_DEADLINE tasks.
> This patches fixes the issue by checking if dl.earliest_dl.curr == 0.
> 
> Signed-off-by: Luca Abeni <luca.abeni@...tn.it>
> ---
>  kernel/sched/deadline.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index fc8f010..0d86d60 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -1066,8 +1066,9 @@ select_task_rq_dl(struct task_struct *p, int cpu, int sd_flag, int flags)
>  		int target = find_later_rq(p);
>  
>  		if (target != -1 &&
> -				dl_time_before(p->dl.deadline,
> -					cpu_rq(target)->dl.earliest_dl.curr))
> +				(dl_time_before(p->dl.deadline,
> +					cpu_rq(target)->dl.earliest_dl.curr) ||
> +				(cpu_rq(target)->dl.earliest_dl.curr == 0)))

Can't we actually use dl.dl_nr_running here and below, so
that we won't incur any wraparound problem?

Thanks,

- Juri

>  			cpu = target;
>  	}
>  	rcu_read_unlock();
> @@ -1417,7 +1418,8 @@ static struct rq *find_lock_later_rq(struct task_struct *task, struct rq *rq)
>  
>  		later_rq = cpu_rq(cpu);
>  
> -		if (!dl_time_before(task->dl.deadline,
> +		if (later_rq->dl.earliest_dl.curr &&
> +			!dl_time_before(task->dl.deadline,
>  					later_rq->dl.earliest_dl.curr)) {
>  			/*
>  			 * Target rq has tasks of equal or earlier deadline,
> 

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