Date: Thu, 21 Apr 2016 16:42:13 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Wanpeng Li <kernellwp@gmail.com>
Cc: Rik van Riel <riel@redhat.com>, Chris Metcalf <cmetcalf@mellanox.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Christoph Lameter <cl@linux.com>, Ingo Molnar <mingo@kernel.org>,
        Luiz Capitulino <lcapitulino@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] nohz_full: Make sched_should_stop_tick() more
 conservative
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On Mon, Apr 18, 2016 at 10:00:42AM +0800, Wanpeng Li wrote:
> > H is for hierarchy. That counts the total of runnable tasks in the
> > entire child hierarchy. Nr_running is the number of se  entities in
> > the current tree.
> 
> So I think we should at least change cfs_rq->nr_running to
> cfs->h_nr_running, I can send a formal patch if you think it makes
> sense. :-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 1159423..79197df 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -616,7 +616,7 @@ bool sched_can_stop_tick(struct rq *rq)
>         }
> 
>         /* Normal multitasking need periodic preemption checks */
> -       if (rq->cfs.nr_running > 1)
> +       if (rq->cfs.h_nr_running > 1)
>                 return false;
> 
>         return true;

So I think that is indeed the right thing here. But looking at this
function I think there's more problems with it.

It seems to assume that if there's FIFO tasks, those will run. This is
incorrect. The FIFO task can have a lower prio than an RR task, in which
case the RR task will run.

So the whole fifo_nr_running test seems misplaced, it should go after
the rr_nr_running tests. That is, only if !rr_nr_running, can we use
fifo_nr_running like this.