Re: [RFC 2/4] sched/core: Set nr_lat_sensitive counter at various scheduler entry/exit points

Pavan Kondeti
2020-05-08T08:33:27Z

Hi Parth,

On Thu, May 07, 2020 at 07:07:21PM +0530, Parth Shah wrote:
> Monitor tasks at:
> 1. wake_up_new_task() - forked tasks
>
> 2. set_task_cpu() - task migrations, Load balancer
>
> 3. __sched_setscheduler() - set/unset latency_nice value
> Increment the nr_lat_sensitive count on the CPU with task marked with
> latency_nice == -20.
> Similarly, decrement the nr_lat_sensitive counter upon re-marking the task
> with >-20 latency_nice task.
>
> 4. finish_task_switch() - dying task
>

> Signed-off-by: Parth Shah <parth@linux.ibm.com>
> ---
> kernel/sched/core.c | 30 ++++++++++++++++++++++++++++--
> kernel/sched/sched.h | 5 +++++
> 2 files changed, 33 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 2d8b76f41d61..ad396c36eba6 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1744,6 +1744,11 @@ void set_task_cpu(struct task_struct *p, unsigned int new_cpu)
> trace_sched_migrate_task(p, new_cpu);
>
> if (task_cpu(p) != new_cpu) {
> + if (task_is_lat_sensitive(p)) {
> + per_cpu(nr_lat_sensitive, task_cpu(p))--;
> + per_cpu(nr_lat_sensitive, new_cpu)++;
> + }
> +

Since we can come here without rq locks, there is a possibility
of a race and incorrect updates can happen. Since the counters
are used to prevent C-states, we don't want that to happen.

> if (p->sched_class->migrate_task_rq)
> p->sched_class->migrate_task_rq(p, new_cpu);
> p->se.nr_migrations++;
> @@ -2947,6 +2952,7 @@ void wake_up_new_task(struct task_struct *p)
> {
> struct rq_flags rf;
> struct rq *rq;
> + int target_cpu = 0;
>
> raw_spin_lock_irqsave(&p->pi_lock, rf.flags);
> p->state = TASK_RUNNING;
> @@ -2960,9 +2966,17 @@ void wake_up_new_task(struct task_struct *p)
> * as we're not fully set-up yet.
> */
> p->recent_used_cpu = task_cpu(p);
> - __set_task_cpu(p, select_task_rq(p, task_cpu(p), SD_BALANCE_FORK, 0));
> + target_cpu = select_task_rq(p, task_cpu(p), SD_BALANCE_FORK, 0);
> + __set_task_cpu(p, target_cpu);
> +

The target_cpu variable can be eliminated by using task_cpu(p) directly
in the below update.

> #endif
> rq = __task_rq_lock(p, &rf);
> +
> +#ifdef CONFIG_SMP
> + if (task_is_lat_sensitive(p))
> + per_cpu(nr_lat_sensitive, target_cpu)++;
> +#endif
> +

Is the SMP check intentional? In some parts of this patch, updates to
nr_lat_sensitive are done without SMP checks. For example,
finish_task_switch() below.

> update_rq_clock(rq);
> post_init_entity_util_avg(p);
>
> @@ -3248,6 +3262,9 @@ static struct rq *finish_task_switch(struct task_struct *prev)
> if (prev->sched_class->task_dead)
> prev->sched_class->task_dead(prev);
>
> + if (task_is_lat_sensitive(prev))
> + per_cpu(nr_lat_sensitive, prev->cpu)--;
> +
> /*
> * Remove function-return probe instances associated with this
> * task and put them back on the free list.
> @@ -4732,8 +4749,17 @@ static void __setscheduler_params(struct task_struct *p,
> p->normal_prio = normal_prio(p);
> set_load_weight(p, true);
>
> - if (attr->sched_flags & SCHED_FLAG_LATENCY_NICE)
> + if (attr->sched_flags & SCHED_FLAG_LATENCY_NICE) {
> + if (p->state != TASK_DEAD &&
> + attr->sched_latency_nice != p->latency_nice) {
> + if (attr->sched_latency_nice == MIN_LATENCY_NICE)
> + per_cpu(nr_lat_sensitive, task_cpu(p))++;
> + else if (task_is_lat_sensitive(p))
> + per_cpu(nr_lat_sensitive, task_cpu(p))--;
> + }
> +
> p->latency_nice = attr->sched_latency_nice;
> + }
> }

There is a potential race here due to which we can mess up the refcount.

- A latency sensitive task is marked TASK_DEAD
<snip>
- sched_setattr() called on the task to clear the latency nice. Since
we check the task state here, we skip the decrement.
- The task is finally context switched out and we skip the decrement again
since it is not a latency senstivie task.

>
> /* Actually do priority change: must hold pi & rq lock. */
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 5c41020c530e..56f885e37451 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -211,6 +211,11 @@ static inline int task_has_dl_policy(struct task_struct *p)
> return dl_policy(p->policy);
> }
>
> +static inline int task_is_lat_sensitive(struct task_struct *p)
> +{
> + return p->latency_nice == MIN_LATENCY_NICE;
> +}
> +
> #define cap_scale(v, s) ((v)*(s) >> SCHED_CAPACITY_SHIFT)
>
> /*
> --
> 2.17.2
>

Thanks,
Pavan

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