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

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Message-ID: <20200428220917.GB16027@hirez.programming.kicks-ass.net>
Date:   Wed, 29 Apr 2020 00:09:17 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     Scott Wood <swood@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        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, Apr 28, 2020 at 10:37:18PM +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.

Also, if you move it this late, this is entirely the wrong place. If you
do it after the context switch either use the balance_callback or put it
in the idle path.

But what Valentin said; this needs a fair bit of support, the whole
reason we've never done this is to avoid that double context switch...