linux-kernel - Re: [PATCH 3/3] sched: Optimize !CONFIG_NO_HZ

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Date:	Mon, 18 Apr 2016 15:36:34 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Byungchul Park <byungchul.park@....com>,
	Chris Metcalf <cmetcalf@...hip.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Luiz Capitulino <lcapitulino@...hat.com>,
	Christoph Lameter <cl@...ux.com>,
	"Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
	Mike Galbraith <efault@....de>, Rik van Riel <riel@...hat.com>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH 3/3] sched: Optimize !CONFIG_NO_HZ_COMMON cpu load updates

On Wed, Apr 13, 2016 at 03:56:52PM +0200, Frederic Weisbecker wrote:
> Some code in cpu load update only concern NO_HZ configs but it is
> built on all configurations. When NO_HZ isn't built, that code is harmless
> but just happens to take some useless ressources in CPU and memory:
> 
> 1) one useless field in struct rq
> 2) jiffies record on every tick that is never used (cpu_load_update_periodic)
> 3) decay_load_missed is called two times on every tick to eventually
>    return immediately with no action taken. And that function is dead
>    code.
> 
> For pure optimization purposes, lets conditionally build the NO_HZ
> related code.
> 
> Cc: Byungchul Park <byungchul.park@....com>
> Cc: Chris Metcalf <cmetcalf@...hip.com>
> Cc: Christoph Lameter <cl@...ux.com>
> Cc: Ingo Molnar <mingo@...e.hu>
> Cc: Luiz Capitulino <lcapitulino@...hat.com>
> Cc: Mike Galbraith <efault@....de>
> Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Rik van Riel <riel@...hat.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> ---
>  kernel/sched/core.c  | 3 ++-
>  kernel/sched/fair.c  | 9 +++++++--
>  kernel/sched/sched.h | 6 ++++--
>  3 files changed, 13 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 4c522a7..59a2821 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -7327,8 +7327,9 @@ void __init sched_init(void)
>  
>  		for (j = 0; j < CPU_LOAD_IDX_MAX; j++)
>  			rq->cpu_load[j] = 0;
> -
> +#ifdef CONFIG_NO_HZ_COMMON
>  		rq->last_load_update_tick = jiffies;
> +#endif

I forgot to also conditionally initialize it in sched_init(), I need to resend
the patchset with that fixed.