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SubjectRe: [PATCH v2 2/4] sched: track the next-highest priority on each runqueue
Gregory Haskins wrote:
> We will use this later in the series to reduce the amount of rq-lock
> contention during a pull operation
>
> Signed-off-by: Gregory Haskins <ghaskins@novell.com>
> ---
>
> kernel/sched.c | 8 ++++-
> kernel/sched_rt.c | 81 ++++++++++++++++++++++++++++++++++++++++-------------
> 2 files changed, 67 insertions(+), 22 deletions(-)
>
> diff --git a/kernel/sched.c b/kernel/sched.c
> index 6237b9b..24b11eb 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -463,7 +463,10 @@ struct rt_rq {
> struct rt_prio_array active;
> unsigned long rt_nr_running;
> #if defined CONFIG_SMP || defined CONFIG_RT_GROUP_SCHED
> - int highest_prio; /* highest queued rt task prio */
> + struct {
> + int curr; /* highest queued rt task prio */
> + int next; /* next highest */
> + } highest_prio;
> #endif
> #ifdef CONFIG_SMP
> unsigned long rt_nr_migratory;
> @@ -8073,7 +8076,8 @@ static void init_rt_rq(struct rt_rq *rt_rq, struct rq *rq)
> __set_bit(MAX_RT_PRIO, array->bitmap);
>
> #if defined CONFIG_SMP || defined CONFIG_RT_GROUP_SCHED
> - rt_rq->highest_prio = MAX_RT_PRIO;
> + rt_rq->highest_prio.curr = MAX_RT_PRIO;
> + rt_rq->highest_prio.next = MAX_RT_PRIO;
> #endif
> #ifdef CONFIG_SMP
> rt_rq->rt_nr_migratory = 0;
> diff --git a/kernel/sched_rt.c b/kernel/sched_rt.c
> index fb1d4d7..a4022b6 100644
> --- a/kernel/sched_rt.c
> +++ b/kernel/sched_rt.c
> @@ -108,7 +108,7 @@ static void sched_rt_rq_enqueue(struct rt_rq *rt_rq)
> if (rt_rq->rt_nr_running) {
> if (rt_se && !on_rt_rq(rt_se))
> enqueue_rt_entity(rt_se);
> - if (rt_rq->highest_prio < curr->prio)
> + if (rt_rq->highest_prio.curr < curr->prio)
> resched_task(curr);
> }
> }
> @@ -473,7 +473,7 @@ static inline int rt_se_prio(struct sched_rt_entity *rt_se)
> struct rt_rq *rt_rq = group_rt_rq(rt_se);
>
> if (rt_rq)
> - return rt_rq->highest_prio;
> + return rt_rq->highest_prio.curr;
> #endif
>
> return rt_task_of(rt_se)->prio;
> @@ -547,6 +547,21 @@ static void update_curr_rt(struct rq *rq)
> }
> }
>
> +#if defined CONFIG_SMP || defined CONFIG_RT_GROUP_SCHED
> +
> +static struct task_struct *pick_next_highest_task_rt(struct rq *rq, int cpu);
> +
> +static inline int next_prio(struct rq *rq)
> +{
> + struct task_struct *next = pick_next_highest_task_rt(rq, rq->cpu);
> +
> + if (next && rt_prio(next->prio))
> + return next->prio;
> + else
> + return MAX_RT_PRIO;
> +}
> +#endif
> +
> static inline
> void inc_rt_tasks(struct sched_rt_entity *rt_se, struct rt_rq *rt_rq)
> {
> @@ -560,14 +575,32 @@ void inc_rt_tasks(struct sched_rt_entity *rt_se, struct rt_rq *rt_rq)
> WARN_ON(!rt_prio(prio));
> rt_rq->rt_nr_running++;
> #if defined CONFIG_SMP || defined CONFIG_RT_GROUP_SCHED
> - if (prio < rt_rq->highest_prio) {
> + if (prio < rt_rq->highest_prio.curr) {
>
> - rt_rq->highest_prio = prio;
> + /*
> + * If the new task is higher in priority than anything on the
> + * run-queue, we have a new high that must be published to
> + * the world. We also know that the previous high becomes
> + * our next-highest.
> + */
> + rt_rq->highest_prio.next = rt_rq->highest_prio.curr;
> + rt_rq->highest_prio.curr = prio;
> #ifdef CONFIG_SMP
> if (rq->online)
> cpupri_set(&rq->rd->cpupri, rq->cpu, prio);
> #endif
> - }
> + } else if (prio == rt_rq->highest_prio.curr)
> + /*
> + * If the next task is equal in priority to the highest on
> + * the run-queue, then we implicitly know that the next highest
> + * task cannot be any lower than current
> + */
> + rt_rq->highest_prio.next = prio;
> + else if (prio < rt_rq->highest_prio.next)
> + /*
> + * Otherwise, we need to recompute next-highest
> + */
> + rt_rq->highest_prio.next = next_prio(rq);
> #endif
> #ifdef CONFIG_SMP
> if (rt_se->nr_cpus_allowed > 1)
> @@ -591,7 +624,7 @@ void dec_rt_tasks(struct sched_rt_entity *rt_se, struct rt_rq *rt_rq)
> {
> #ifdef CONFIG_SMP
> struct rq *rq = rq_of_rt_rq(rt_rq);
> - int highest_prio = rt_rq->highest_prio;
> + int highest_prio = rt_rq->highest_prio.curr;
> #endif
>
> WARN_ON(!rt_prio(rt_se_prio(rt_se)));
> @@ -599,24 +632,32 @@ void dec_rt_tasks(struct sched_rt_entity *rt_se, struct rt_rq *rt_rq)
> rt_rq->rt_nr_running--;
> #if defined CONFIG_SMP || defined CONFIG_RT_GROUP_SCHED
> if (rt_rq->rt_nr_running) {
> - struct rt_prio_array *array;
> + int prio = rt_se_prio(rt_se);
> +
> + WARN_ON(prio < rt_rq->highest_prio.curr);
>
> - WARN_ON(rt_se_prio(rt_se) < rt_rq->highest_prio);
> - if (rt_se_prio(rt_se) == rt_rq->highest_prio) {
> - /* recalculate */
> - array = &rt_rq->active;
> - rt_rq->highest_prio =
> + /*
> + * This may have been our highest or next-highest priority
> + * task and therefore we may have some recomputation to do
> + */
> + if (prio == rt_rq->highest_prio.curr) {
> + struct rt_prio_array *array = &rt_rq->active;
> +
> + rt_rq->highest_prio.curr =
> sched_find_first_bit(array->bitmap);
> - } /* otherwise leave rq->highest prio alone */
> + }
> +
> + if (prio == rt_rq->highest_prio.next)
>

Crap. Trying to fall asleep tonight, I realized this is a bug I think.
Looks like I will need a v3

It should be "prio <= rt_rq->highest_prio.next" or we can miss updating
.next properly.

> + rt_rq->highest_prio.next = next_prio(rq);
> } else
> - rt_rq->highest_prio = MAX_RT_PRIO;
> + rt_rq->highest_prio.curr = MAX_RT_PRIO;
> #endif
> #ifdef CONFIG_SMP
> if (rt_se->nr_cpus_allowed > 1)
> rq->rt.rt_nr_migratory--;
>
> - if (rq->online && rt_rq->highest_prio != highest_prio)
> - cpupri_set(&rq->rd->cpupri, rq->cpu, rt_rq->highest_prio);
> + if (rq->online && rt_rq->highest_prio.curr != highest_prio)
> + cpupri_set(&rq->rd->cpupri, rq->cpu, rt_rq->highest_prio.curr);
>
> update_rt_migration(rq);
> #endif /* CONFIG_SMP */
> @@ -1066,7 +1107,7 @@ static struct rq *find_lock_lowest_rq(struct task_struct *task, struct rq *rq)
> }
>
> /* If this rq is still suitable use it. */
> - if (lowest_rq->rt.highest_prio > task->prio)
> + if (lowest_rq->rt.highest_prio.curr > task->prio)
> break;
>
> /* try again */
> @@ -1254,7 +1295,7 @@ static int pull_rt_task(struct rq *this_rq)
> static void pre_schedule_rt(struct rq *rq, struct task_struct *prev)
> {
> /* Try to pull RT tasks here if we lower this rq's prio */
> - if (unlikely(rt_task(prev)) && rq->rt.highest_prio > prev->prio)
> + if (unlikely(rt_task(prev)) && rq->rt.highest_prio.curr > prev->prio)
> pull_rt_task(rq);
> }
>
> @@ -1340,7 +1381,7 @@ static void rq_online_rt(struct rq *rq)
>
> __enable_runtime(rq);
>
> - cpupri_set(&rq->rd->cpupri, rq->cpu, rq->rt.highest_prio);
> + cpupri_set(&rq->rd->cpupri, rq->cpu, rq->rt.highest_prio.curr);
> }
>
> /* Assumes rq->lock is held */
> @@ -1431,7 +1472,7 @@ static void prio_changed_rt(struct rq *rq, struct task_struct *p,
> * can release the rq lock and p could migrate.
> * Only reschedule if p is still on the same runqueue.
> */
> - if (p->prio > rq->rt.highest_prio && rq->curr == p)
> + if (p->prio > rq->rt.highest_prio.curr && rq->curr == p)
> resched_task(p);
> #else
> /* For UP simply resched on drop of prio */
>
> --
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