linux-kernel - Re: [RFC PATCH 1/2] sched/rt: Disable runtime accounting for idle threads with SCHED

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Date: Wed, 10 Apr 2024 09:00:23 +0200
From: Daniel Bristot de Oliveira <bristot@...hat.com>
To: Atul Pant <quic_atulpant@...cinc.com>, mingo@...hat.com,
 peterz@...radead.org, juri.lelli@...hat.com, vincent.guittot@...aro.org,
 dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
 mgorman@...e.de, vschneid@...hat.com, rafael@...nel.org,
 daniel.lezcano@...aro.org
Cc: kernel@...cinc.com, linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org
Subject: Re: [RFC PATCH 1/2] sched/rt: Disable runtime accounting for idle
 threads with SCHED_FIFO policy

On 4/10/24 06:54, Atul Pant wrote:
> To prevent the throttling of RT idle threads, like the idle-inject
> threads, skip accounting of runtime for these threads.
> 
> Signed-off-by: Atul Pant <quic_atulpant@...cinc.com>
> ---
>  kernel/sched/rt.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index 4ac36eb4cdee..d20999270e75 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -1075,7 +1075,9 @@ static void update_curr_rt(struct rq *rq)
>  		struct rt_rq *rt_rq = rt_rq_of_se(rt_se);
>  		int exceeded;
>  
> -		if (sched_rt_runtime(rt_rq) != RUNTIME_INF) {
> +		if (sched_rt_runtime(rt_rq) != RUNTIME_INF &&
> +				!(curr->policy == SCHED_FIFO &&
> +					curr->flags & PF_IDLE)) {


FYI, this will not be a problem with dl server because play_idle_precise()
disables preemption, so the dl server will not be scheduled until preempt_enable().

with the DL server, the time consumed as an rt task will not change the DL server
behavior because the logic inverts: it provides bandwidth for the fair
scheduler (instead of throttling the RT sched).

-- Daniel

>  			raw_spin_lock(&rt_rq->rt_runtime_lock);
>  			rt_rq->rt_time += delta_exec;
>  			exceeded = sched_rt_runtime_exceeded(rt_rq);