linux-kernel - Re: [PATCH V6 1/6] sched/fair: Add trivial fair server

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Date: Wed, 10 Apr 2024 19:24:23 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Daniel Bristot de Oliveira <bristot@...nel.org>
Cc: Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Daniel Bristot de Oliveira <bristot@...hat.com>,
	Valentin Schneider <vschneid@...hat.com>,
	linux-kernel@...r.kernel.org,
	Luca Abeni <luca.abeni@...tannapisa.it>,
	Tommaso Cucinotta <tommaso.cucinotta@...tannapisa.it>,
	Thomas Gleixner <tglx@...utronix.de>,
	Joel Fernandes <joel@...lfernandes.org>,
	Vineeth Pillai <vineeth@...byteword.org>,
	Shuah Khan <skhan@...uxfoundation.org>,
	Phil Auld <pauld@...hat.com>,
	Suleiman Souhlal <suleiman@...gle.com>,
	Youssef Esmat <youssefesmat@...gle.com>
Subject: Re: [PATCH V6 1/6] sched/fair: Add trivial fair server

On Fri, Apr 05, 2024 at 07:28:00PM +0200, Daniel Bristot de Oliveira wrote:
> From: Peter Zijlstra <peterz@...radead.org>
> 
> Use deadline servers to service fair tasks.
> 
> This patch adds a fair_server deadline entity which acts as a container
> for fair entities and can be used to fix starvation when higher priority
> (wrt fair) tasks are monopolizing CPU(s).
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> Signed-off-by: Daniel Bristot de Oliveira <bristot@...nel.org>
> ---
>  kernel/sched/core.c     | 24 ++++++++++++++++--------
>  kernel/sched/deadline.c | 23 +++++++++++++++++++++++
>  kernel/sched/fair.c     | 25 +++++++++++++++++++++++++
>  kernel/sched/sched.h    |  4 ++++
>  4 files changed, 68 insertions(+), 8 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 7019a40457a6..04e2270487b7 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -6007,6 +6007,14 @@ static void put_prev_task_balance(struct rq *rq, struct task_struct *prev,
>  #endif
>  
>  	put_prev_task(rq, prev);
> +
> +	/*
> +	 * We've updated @prev and no longer need the server link, clear it.
> +	 * Must be done before ->pick_next_task() because that can (re)set
> +	 * ->dl_server.
> +	 */
> +	if (prev->dl_server)
> +		prev->dl_server = NULL;
>  }
>  
>  /*
> @@ -6037,6 +6045,13 @@ __pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
>  			p = pick_next_task_idle(rq);
>  		}
>  
> +		/*
> +		 * This is a normal CFS pick, but the previous could be a DL pick.
> +		 * Clear it as previous is no longer picked.
> +		 */
> +		if (prev->dl_server)
> +			prev->dl_server = NULL;
> +
>  		/*
>  		 * This is the fast path; it cannot be a DL server pick;
>  		 * therefore even if @p == @prev, ->dl_server must be NULL.
> @@ -6050,14 +6065,6 @@ __pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
>  restart:
>  	put_prev_task_balance(rq, prev, rf);
>  
> -	/*
> -	 * We've updated @prev and no longer need the server link, clear it.
> -	 * Must be done before ->pick_next_task() because that can (re)set
> -	 * ->dl_server.
> -	 */
> -	if (prev->dl_server)
> -		prev->dl_server = NULL;
> -
>  	for_each_class(class) {
>  		p = class->pick_next_task(rq);
>  		if (p)

This bit seems like a fix for 63ba8422f876 ("sched/deadline: Introduce
deadline servers"), should it be a separate patch?