linux-kernel - Re: [PATCH v2 2/2] sched/deadline: Initialize cp->elements[].cpu to an invalid value

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Message-ID: <20170918084108.GB5994@X58A-UD3R>
Date:   Mon, 18 Sep 2017 17:41:08 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     peterz@...radead.org, mingo@...nel.org, rostedt@...dmis.org
Cc:     linux-kernel@...r.kernel.org, juri.lelli@...il.com,
        kernel-team@....com
Subject: Re: [PATCH v2 2/2] sched/deadline: Initialize cp->elements[].cpu to
 an invalid value

On Thu, Sep 14, 2017 at 10:05:19AM +0900, Byungchul Park wrote:
> Currently, migrating tasks to cpu0 unconditionally happens when the
> heap is empty, since cp->elements[].cpu was initialized to 0(=cpu0).
> We have to distinguish between the empty case and cpu0 to avoid the
> unnecessary migrantions. Therefore, it has to return an invalid value
> e.i. -1 in that case.

I think, the patch is necessary, isn't it?

> Signed-off-by: Byungchul Park <byungchul.park@....com>
> ---
>  kernel/sched/cpudeadline.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/cpudeadline.c b/kernel/sched/cpudeadline.c
> index 9f02035..bcf903f 100644
> --- a/kernel/sched/cpudeadline.c
> +++ b/kernel/sched/cpudeadline.c
> @@ -138,6 +138,12 @@ int cpudl_find(struct cpudl *cp, struct task_struct *p,
>  		int best_cpu = cpudl_maximum_cpu(cp);
>  		WARN_ON(best_cpu != -1 && !cpu_present(best_cpu));
>  
> +		/*
> +		 * The heap tree is empry for now, just return.
> +		 */
> +		if (best_cpu == -1)
> +			return 0;
> +
>  		if (cpumask_test_cpu(best_cpu, &p->cpus_allowed) &&
>  		    dl_time_before(dl_se->deadline, cpudl_maximum_dl(cp))) {
>  			if (later_mask)
> @@ -265,8 +271,10 @@ int cpudl_init(struct cpudl *cp)
>  		return -ENOMEM;
>  	}
>  
> -	for_each_possible_cpu(i)
> +	for_each_possible_cpu(i) {
> +		cp->elements[i].cpu = -1;
>  		cp->elements[i].idx = IDX_INVALID;
> +	}
>  
>  	return 0;
>  }
> -- 
> 1.9.1