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From:   Valentin Schneider <valentin.schneider@arm.com>
To:     Scott Wood <swood@redhat.com>
Cc:     Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Rik van Riel <riel@surriel.com>,
        Mel Gorman <mgorman@suse.de>, linux-kernel@vger.kernel.org,
        linux-rt-users <linux-rt-users@vger.kernel.org>
Subject: Re: [RFC PATCH 1/3] sched/fair: Call newidle_balance() from finish_task_switch()
In-reply-to: <20200428050242.17717-2-swood@redhat.com>
Date:   Tue, 28 Apr 2020 22:37:18 +0100
Message-ID: <jhjftcns35d.mognet@arm.com>
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Sender: linux-kernel-owner@vger.kernel.org
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On 28/04/20 06:02, Scott Wood wrote:
> Thus, newidle_balance() is entered with interrupts enabled, which allows
> (in the next patch) enabling interrupts when the lock is dropped.
>
> Signed-off-by: Scott Wood <swood@redhat.com>
> ---
>  kernel/sched/core.c  |  7 ++++---
>  kernel/sched/fair.c  | 45 ++++++++++++++++----------------------------
>  kernel/sched/sched.h |  6 ++----
>  3 files changed, 22 insertions(+), 36 deletions(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 9a2fbf98fd6f..0294beb8d16c 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3241,6 +3241,10 @@ static struct rq *finish_task_switch(struct task_struct *prev)
>       }
>
>       tick_nohz_task_switch();
> +
> +	if (is_idle_task(current))
> +		newidle_balance();
> +

This means we must go through a switch_to(idle) before figuring out we
could've switched to a CFS task, and do it then. I'm curious to see the
performance impact of that.

>       return rq;
>  }
>
> @@ -10425,14 +10408,23 @@ static inline void nohz_newidle_balance(struct rq *this_rq) { }
>   *     0 - failed, no new tasks
>   *   > 0 - success, new (fair) tasks present
>   */
> -int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
> +int newidle_balance(void)
>  {
>       unsigned long next_balance = jiffies + HZ;
> -	int this_cpu = this_rq->cpu;
> +	int this_cpu;
>       struct sched_domain *sd;
> +	struct rq *this_rq;
>       int pulled_task = 0;
>       u64 curr_cost = 0;
>
> +	preempt_disable();
> +	this_rq = this_rq();
> +	this_cpu = this_rq->cpu;
> +	local_bh_disable();
> +	raw_spin_lock_irq(&this_rq->lock);
> +
> +	update_rq_clock(this_rq);
> +
>       update_misfit_status(NULL, this_rq);

I'm thinking this should be moved to where newidle_balance() used to be,
otherwise we have a window where the rq is flagged as having a misfit
task despite not having any runnable CFS tasks.

>       /*
>        * We must set idle_stamp _before_ calling idle_balance(), such that we