linux-kernel - Re: [PATCH 2/2] sched/deadline: Fix wrong cpudl_find() in check_preempt_equal

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Date:	Mon, 17 Nov 2014 15:15:15 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	"pang.xunlei" <pang.xunlei@...aro.org>
Cc:	linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Juri Lelli <juri.lelli@...il.com>
Subject: Re: [PATCH 2/2] sched/deadline: Fix wrong cpudl_find() in
 check_preempt_equal_dl()

On Tue, 18 Nov 2014 00:11:04 +0800
"pang.xunlei" <pang.xunlei@...aro.org> wrote:

> In check_preempt_equal_dl(), cpudl_find() is called with a NULL later_mask,
> thus cpudl_find() here doesn't check cpudl.free_cpus at all.
> 
> This patch takles this issue by always passing a non-NULL cpumask to cpudl_find(),
> and assigns later_mask in this function.
> 
> Signed-off-by: pang.xunlei <pang.xunlei@...aro.org>
> ---
>  kernel/sched/cpudeadline.c | 14 ++++++--------
>  kernel/sched/deadline.c    | 10 ++++++----
>  2 files changed, 12 insertions(+), 12 deletions(-)
> 
> diff --git a/kernel/sched/cpudeadline.c b/kernel/sched/cpudeadline.c
> index 9a69353..623f7b5 100755
> --- a/kernel/sched/cpudeadline.c
> +++ b/kernel/sched/cpudeadline.c
> @@ -97,7 +97,7 @@ static inline int cpudl_maximum(struct cpudl *cp)
>   * cpudl_find - find the best (later-dl) CPU in the system
>   * @cp: the cpudl max-heap context
>   * @p: the task
> - * @later_mask: a mask to fill in with the selected CPUs (or NULL)
> + * @later_mask: a mask to fill in with the selected CPUs (not NULL)
>   *
>   * Returns: int - best CPU (heap maximum if suitable)
>   */
> @@ -107,16 +107,14 @@ int cpudl_find(struct cpudl *cp, struct task_struct *p,
>  	int best_cpu = -1;
>  	const struct sched_dl_entity *dl_se = &p->dl;
>  
> -	if (later_mask && cpumask_and(later_mask, cp->free_cpus,
> -			&p->cpus_allowed) && cpumask_and(later_mask,
> -			later_mask, cpu_active_mask)) {
> +	cpumask_and(later_mask, cpu_active_mask, &p->cpus_allowed);
> +	if (cpumask_and(later_mask, later_mask, cp->free_cpus)) {
>  		best_cpu = cpumask_any(later_mask);
>  		goto out;
> -	} else if (cpumask_test_cpu(cpudl_maximum(cp), &p->cpus_allowed) &&
> -			dl_time_before(dl_se->deadline, cp->elements[0].dl)) {
> +	} else if (cpumask_and(later_mask, cpumask_of(cpudl_maximum(cp)),
> +			&p->cpus_allowed) && dl_time_before(dl_se->deadline,
> +			cp->elements[0].dl)) {
>  		best_cpu = cpudl_maximum(cp);
> -		if (later_mask)
> -			cpumask_set_cpu(best_cpu, later_mask);
>  	}
>  

OK, I need to make this a before/after, as patch format sucks in this
case.

before:

	if (later_mask &&
	    cpumask_and(later_mask, cp->free_cpus, &p->cpus_allowed) &&
	    cpumask_and(later_mask, later_mask, cpu_active_mask)) {
		best_cpu = cpumask_any(later_mask);
		goto out;
	} else if (cpumask_test_cpu(cpudl_maximum(cp), &p->cpus_allowed) &&
		dl_time_before(dl_se->deadline, cp->elements[0].dl)) {
		best_cpu = cpudl_maximum(cp);
		if (later_mask)
			cpumask_set_cpu(best_cpu, later_mask);
	}

after:

	cpumask_and(later_mask, cpu_active_mask, &p->cpus_allowed);
	if (cpumask_and(later_mask, later_mask, cp->free_cpus)) {
		best_cpu = cpumask_any(later_mask);
		goto out;
	} else if (cpumask_and(later_mask, cpumask_of(cpudl_maximum(cp)),
			&p->cpus_allowed) && dl_time_before(dl_se->deadline,
			cp->elements[0].dl)) {
		best_cpu = cpudl_maximum(cp);
	}

Again, the above else if is rather ugly. Note, if the first
cpumask_and() is zero, there's no reason to call cpumask_and() (which
could be expensive) the second time. Keep the two together with &&.

Also, I wouldn't convert the cpumask_test_cpu() to a cpumask_and(), as
I would think that would be more expensive to copy an entire mask
(think what happens when we have a thousand CPUs). Just doing a test,
and then if the test succeeds (remember, there's another test that
might fail after it), then just set the one bit.

--- Steve



>  out:
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index bd83272..3ecf838 100755
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -965,14 +965,18 @@ out:
>  	return cpu;
>  }
>  
> +static DEFINE_PER_CPU(cpumask_var_t, local_cpu_mask_dl);
> +
>  static void check_preempt_equal_dl(struct rq *rq, struct task_struct *p)
>  {
> +	struct cpumask *later_mask = this_cpu_cpumask_var_ptr(local_cpu_mask_dl);
> +
>  	/*
>  	 * Current can't be migrated, useless to reschedule,
>  	 * let's hope p can move out.
>  	 */
>  	if (rq->curr->nr_cpus_allowed == 1 ||
> -	    cpudl_find(&rq->rd->cpudl, rq->curr, NULL) == -1)
> +	    cpudl_find(&rq->rd->cpudl, rq->curr, later_mask) == -1)
>  		return;
>  
>  	/*
> @@ -980,7 +984,7 @@ static void check_preempt_equal_dl(struct rq *rq, struct task_struct *p)
>  	 * see if it is pushed or pulled somewhere else.
>  	 */
>  	if (p->nr_cpus_allowed != 1 &&
> -	    cpudl_find(&rq->rd->cpudl, p, NULL) != -1)
> +	    cpudl_find(&rq->rd->cpudl, p, later_mask) != -1)
>  		return;
>  
>  	resched_curr(rq);
> @@ -1167,8 +1171,6 @@ next_node:
>  	return NULL;
>  }
>  
> -static DEFINE_PER_CPU(cpumask_var_t, local_cpu_mask_dl);
> -
>  static int find_later_rq(struct task_struct *task)
>  {
>  	struct sched_domain *sd;

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