linux-kernel - Re: [PATCH] kernel/sched/core.c: Avoid unused variable on non-SMP configs

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Date:   Sun, 9 Sep 2018 11:36:11 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kernel/sched/core.c: Avoid unused variable on non-SMP
 configs

On Sat, Sep 08, 2018 at 09:05:53PM +0200, Miguel Ojeda wrote:
> On non-SMP configs, when only one of CONFIG_{PARAVIRT,IRQ_TIME}_ACCOUNTING
> is defined, we are declaring a variable (irq_delta or steal) which
> is not used:
> 
>     kernel/sched/core.c: In function ‘update_rq_clock_task’:
>     kernel/sched/core.c:139:17: warning: unused variable ‘irq_delta’ [-Wunused-variable]
>       s64 steal = 0, irq_delta = 0;
> 
> The reason is that CONFIG_SMP guards HAVE_SCHED_AVG_IRQ, which in turn
> disables the code guarded by HAVE_SCHED_AVG_IRQ.
> 
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Signed-off-by: Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
> ---
>  kernel/sched/core.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 625bc9897f62..d662d1e11843 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -135,8 +135,11 @@ static void update_rq_clock_task(struct rq *rq, s64 delta)
>   * In theory, the compile should just see 0 here, and optimize out the call
>   * to sched_rt_avg_update. But I don't trust it...
>   */
> -#if defined(CONFIG_IRQ_TIME_ACCOUNTING) || defined(CONFIG_PARAVIRT_TIME_ACCOUNTING)
> -	s64 steal = 0, irq_delta = 0;
> +#if defined(HAVE_SCHED_AVG_IRQ) || defined(CONFIG_IRQ_TIME_ACCOUNTING)
> +	s64 irq_delta = 0;
> +#endif
> +#if defined(HAVE_SCHED_AVG_IRQ) || defined(CONFIG_PARAVIRT_TIME_ACCOUNTING)
> +	s64 steal = 0;
>  #endif
>  #ifdef CONFIG_IRQ_TIME_ACCOUNTING
>  	irq_delta = irq_time_read(cpu_of(rq)) - rq->prev_irq_time;
> -- 

Alternatively, if you don't want to let the crazy ifdeffery in that
function grow even more, you can simply do:

---
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 625bc9897f62..1728743360d4 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -136,7 +136,7 @@ static void update_rq_clock_task(struct rq *rq, s64 delta)
  * to sched_rt_avg_update. But I don't trust it...
  */
 #if defined(CONFIG_IRQ_TIME_ACCOUNTING) || defined(CONFIG_PARAVIRT_TIME_ACCOUNTING)
-	s64 steal = 0, irq_delta = 0;
+	s64 steal __maybe_unused = 0, irq_delta = 0;
 #endif
 #ifdef CONFIG_IRQ_TIME_ACCOUNTING
 	irq_delta = irq_time_read(cpu_of(rq)) - rq->prev_irq_time;

-- 
Regards/Gruss,
    Boris.

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