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Date: Mon, 7 Nov 2016 11:31:47 +0100
From: Tommaso Cucinotta <tommaso.cucinotta@...up.it>
To: Daniel Bristot de Oliveira <bristot@...hat.com>,
Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>
Cc: Steven Rostedt <rostedt@...dmis.org>,
Christoph Lameter <cl@...ux.com>,
linux-rt-users <linux-rt-users@...r.kernel.org>,
LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched/rt: RT_RUNTIME_GREED sched feature
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@...hat.com>
> Reviewed-by: Steven Rostedt <rostedt@...dmis.org>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Christoph Lameter <cl@...ux.com>
> Cc: linux-rt-users <linux-rt-users@...r.kernel.org>
> Cc: LKML <linux-kernel@...r.kernel.org>
>
> 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 */
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