linux-kernel - Re: [RFC PATCH 1/3] sched/fair: Call newidle_balance() from finish_task

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Date:   Tue, 28 Apr 2020 17:33:01 -0500
From:   Scott Wood <swood@...hat.com>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Rik van Riel <riel@...riel.com>,
        Mel Gorman <mgorman@...e.de>, linux-kernel@...r.kernel.org,
        linux-rt-users <linux-rt-users@...r.kernel.org>
Subject: Re: [RFC PATCH 1/3] sched/fair: Call newidle_balance() from
 finish_task_switch()

On Tue, 2020-04-28 at 22:37 +0100, Valentin Schneider wrote:
> 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@...hat.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.

Any particular benchmark I should try?

> >      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.

OK

-Scott