linux-kernel - Re: [PATCH 4/4] nohz: Convert a few places to use local per cpu accesses

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Date:	Fri, 16 Aug 2013 18:00:43 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Fernando Luis Vazquez Cao <fernando_b1@....ntt.co.jp>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arjan van de Ven <arjan@...ux.intel.com>
Subject: Re: [PATCH 4/4] nohz: Convert a few places to use local per cpu
 accesses

OK so these patches look ok to me -- didn't read in detail though.

On Fri, Aug 16, 2013 at 05:42:33PM +0200, Frederic Weisbecker wrote:
> A few functions use remote per CPU access APIs when they
> deal with local values.
> 
> Just to the right conversion to improve performance, code
> readability and debug checks.

> @@ -410,7 +408,7 @@ static void tick_nohz_update_jiffies(ktime_t now)
>  
>  static void tick_nohz_stop_idle(int cpu, ktime_t now)
>  {
> -	struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
> +	struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched);

What's there from stopping someone from calling this with cpu !=
smp_processor_id() ? 

> @@ -1020,7 +1018,7 @@ static void tick_nohz_kick_tick(int cpu, ktime_t now)
>  
>  static inline void tick_check_nohz(int cpu)
>  {
> -	struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
> +	struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched);
>  	ktime_t now;
>  
>  	if (!ts->idle_active && !ts->tick_stopped)

idem
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