linux-kernel - Re: [PATCH 16/32] nohz/cpuset: Wake up adaptive nohz CPU when a timer gets enqueued

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Date:	Mon, 29 Aug 2011 17:51:03 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Anton Blanchard <anton@....ibm.com>,
	Avi Kivity <avi@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	"Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
	Paul Menage <menage@...gle.com>,
	Stephen Hemminger <shemminger@...tta.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Tim Pepper <lnxninja@...ux.vnet.ibm.com>
Subject: Re: [PATCH 16/32] nohz/cpuset: Wake up adaptive nohz CPU when a
 timer gets enqueued

On Mon, 2011-08-15 at 17:52 +0200, Frederic Weisbecker wrote:
> Wake up a CPU when a timer list timer is enqueued there and
> the CPU is in adaptive nohz mode. Sending an IPI to it makes
> it reconsidering the next timer to program on top of recent
> updates.
> 

>  include/linux/sched.h    |    4 ++--
>  kernel/sched.c           |   33 ++++++++++++++++++++++++++++++++-
>  kernel/time/tick-sched.c |    5 ++++-
>  kernel/timer.c           |    2 +-
>  4 files changed, 39 insertions(+), 5 deletions(-) 

So here I would have expected timer_needs_cpu() and an addition to
tick_nohz_can_stop_tick(). Why does sched.c get touched at all?

Also, all the delta_jiffies stuff in the current
tick_nohz_stop_sched_tick() deals with this, why duplicate the logic?
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