linux-kernel - Re: [PATCH 4/7] sched: Account rr and fifo tasks separately

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Date:	Wed, 2 Dec 2015 13:53:39 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Chris Metcalf <cmetcalf@...hip.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Luiz Capitulino <lcapitulino@...hat.com>,
	Christoph Lameter <cl@...ux.com>,
	Ingo Molnar <mingo@...nel.org>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH 4/7] sched: Account rr and fifo tasks separately

On Fri, Nov 13, 2015 at 03:22:06PM +0100, Frederic Weisbecker wrote:
> In order to evaluate tick dependency, we need to account SCHED_RR and
> SCHED_FIFO tasks separately as those policies don't have the same
> preemption requirements.
> 
> We still keep rt_nr_running as a cache to avoid additions between nr_rr
> and nr_fifo all over the place.

In which case you only need one of nr_fifo/nr_rr. Less accounting is
better.

Pick the one you need for the nohz_full condition, and leave the other.
A quick look at sched_can_stop_tick() seems to suggest nr_rr is the
interesting one. nr_rr < 2 should allow stopping the tick.
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