Re: [PATCH] sched/rt: RT_RUNTIME_GREED sched feature

[Tommaso Cucinotta]

as anticipated live to Daniel:
-) +1 for the general concept, we'd need something similar also for SCHED_DEADLINE
-) only issue might be that, if a non-RT task wakes up after the unthrottle, it will have to wait, but worst-case it will have a chance in the next throttling window
-) an alternative to unthrottling might be temporary class downgrade to sched_other, but that might be much more complex, instead this Daniel's one looks quite simple
-) when considering also DEADLINE tasks, it might be good to think about how we'd like the throttling of DEADLINE and RT tasks to inter-relate, e.g.:
  a) DEADLINE unthrottles if there's no RT nor OTHER tasks? what if there's an unthrottled RT?
  b) DEADLINE throttles by downgrading to OTHER?
  c) DEADLINE throttles by downgrading to RT (RR/FIFO and what prio?)

My2c, thanks!

    T.

On 07/11/2016 09:17, Daniel Bristot de Oliveira wrote:
> The rt throttling mechanism prevents the starvation of non-real-time
> tasks by CPU intensive real-time tasks. In terms of percentage,
> the default behavior allows real-time tasks to run up to 95% of a
> given period, leaving the other 5% of the period for non-real-time
> tasks. In the absence of non-rt tasks, the system goes idle for 5%
> of the period.
>
> Although this behavior works fine for the purpose of avoiding
> bad real-time tasks that can hang the system, some greed users
> want to allow the real-time task to continue running in the absence
> of non-real-time tasks starving. In other words, they do not want to
> see the system going idle.
>
> This patch implements the RT_RUNTIME_GREED scheduler feature for greedy
> users (TM). When enabled, this feature will check if non-rt tasks are
> starving before throttling the real-time task. If the real-time task
> becomes throttled, it will be unthrottled as soon as the system goes
> idle, or when the next period starts, whichever comes first.
>
> This feature is enabled with the following command:
>    # echo RT_RUNTIME_GREED > /sys/kernel/debug/sched_features
>
> The user might also want to disable NO_RT_RUNTIME_SHARE logic,
> to keep all CPUs with the same rt_runtime.
>    # echo NO_RT_RUNTIME_SHARE > /sys/kernel/debug/sched_features
>
> With these two options set, the user will guarantee some runtime
> for non-rt-tasks on all CPUs, while keeping real-time tasks running
> as much as possible.
>
> The feature is disabled by default, keeping the current behavior.
>
> Signed-off-by: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>
> Reviewed-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Cc: Christoph Lameter <cl@xxxxxxxxx>
> Cc: linux-rt-users <linux-rt-users@xxxxxxxxxxxxxxx>
> Cc: LKML <linux-kernel@xxxxxxxxxxxxxxx>
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 42d4027..c4c62ee 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3275,7 +3275,8 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct pin_cookie cookie
>   		if (unlikely(!p))
>   			p = idle_sched_class.pick_next_task(rq, prev, cookie);
>   
> -		return p;
> +		if (likely(p != RETRY_TASK))
> +			return p;
>   	}
>   
>   again:
> diff --git a/kernel/sched/features.h b/kernel/sched/features.h
> index 69631fa..3bd7a6d 100644
> --- a/kernel/sched/features.h
> +++ b/kernel/sched/features.h
> @@ -66,6 +66,7 @@ SCHED_FEAT(RT_PUSH_IPI, true)
>   
>   SCHED_FEAT(FORCE_SD_OVERLAP, false)
>   SCHED_FEAT(RT_RUNTIME_SHARE, true)
> +SCHED_FEAT(RT_RUNTIME_GREED, false)
>   SCHED_FEAT(LB_MIN, false)
>   SCHED_FEAT(ATTACH_AGE_LOAD, true)
>   
> diff --git a/kernel/sched/idle_task.c b/kernel/sched/idle_task.c
> index 5405d3f..0f23e06 100644
> --- a/kernel/sched/idle_task.c
> +++ b/kernel/sched/idle_task.c
> @@ -26,6 +26,10 @@ static void check_preempt_curr_idle(struct rq *rq, struct task_struct *p, int fl
>   static struct task_struct *
>   pick_next_task_idle(struct rq *rq, struct task_struct *prev, struct pin_cookie cookie)
>   {
> +	if (sched_feat(RT_RUNTIME_GREED))
> +		if (try_to_unthrottle_rt_rq(&rq->rt))
> +			return RETRY_TASK;
> +
>   	put_prev_task(rq, prev);
>   	update_idle_core(rq);
>   	schedstat_inc(rq->sched_goidle);
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index 2516b8d..a6961a5 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -631,6 +631,22 @@ static inline struct rt_bandwidth *sched_rt_bandwidth(struct rt_rq *rt_rq)
>   
>   #endif /* CONFIG_RT_GROUP_SCHED */
>   
> +static inline void unthrottle_rt_rq(struct rt_rq *rt_rq)
> +{
> +	rt_rq->rt_time = 0;
> +	rt_rq->rt_throttled = 0;
> +	sched_rt_rq_enqueue(rt_rq);
> +}
> +
> +int try_to_unthrottle_rt_rq(struct rt_rq *rt_rq)
> +{
> +	if (rt_rq_throttled(rt_rq)) {
> +		unthrottle_rt_rq(rt_rq);
> +		return 1;
> +	}
> +	return 0;
> +}
> +
>   bool sched_rt_bandwidth_account(struct rt_rq *rt_rq)
>   {
>   	struct rt_bandwidth *rt_b = sched_rt_bandwidth(rt_rq);
> @@ -920,6 +936,18 @@ static int sched_rt_runtime_exceeded(struct rt_rq *rt_rq)
>   		 * but accrue some time due to boosting.
>   		 */
>   		if (likely(rt_b->rt_runtime)) {
> +			if (sched_feat(RT_RUNTIME_GREED)) {
> +				struct rq *rq = rq_of_rt_rq(rt_rq);
> +				/*
> +				 * If there is no other tasks able to run
> +				 * on this rq, lets be greed and reset our
> +				 * rt_time.
> +				 */
> +				if (rq->nr_running == rt_rq->rt_nr_running) {
> +					rt_rq->rt_time = 0;
> +					return 0;
> +				}
> +			}
>   			rt_rq->rt_throttled = 1;
>   			printk_deferred_once("sched: RT throttling activated\n");
>   		} else {
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 055f935..450ca34 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -502,6 +502,8 @@ struct rt_rq {
>   #endif
>   };
>   
> +int try_to_unthrottle_rt_rq(struct rt_rq *rt_rq);
> +
>   /* Deadline class' related fields in a runqueue */
>   struct dl_rq {
>   	/* runqueue is an rbtree, ordered by deadline */