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

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Date:	Mon, 11 Apr 2016 10:53:01 -0400
From:	Chris Metcalf <cmetcalf@...lanox.com>
To:	Frederic Weisbecker <fweisbec@...il.com>,
	Peter Zijlstra <peterz@...radead.org>
CC:	LKML <linux-kernel@...r.kernel.org>,
	Byungchul Park <byungchul.park@....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 4/11/2016 9:18 AM, Frederic Weisbecker wrote:
> So I tried and it warns about the unused variable tickless_load, so I
> would need two scattered ifdeffery in the function:
>
> @@ -4528,7 +4529,9 @@ decay_load_missed(unsigned long load, unsigned long missed_updates, int idx)
>   static void cpu_load_update(struct rq *this_rq, unsigned long this_load,
>   			    unsigned long pending_updates)
>   {
> +#ifdef CONFIG_NO_HZ_COMMON
>   	unsigned long tickless_load = this_rq->cpu_load[0];
> +#endif

Just move the initialization down to the first use, as a regular
assignment, and add __maybe_unused to the declaration, and the compiler
will then keep quiet (see Documentation/CodingStyle).

I have no comment on which of the approaches looks better overall,
but I think using __maybe_unused definitely improves this approach.
-- 
Chris Metcalf, Mellanox Technologies
http://www.mellanox.com