linux-kernel - Re: [PATCH 09/10] sched: task_sched_runtime introduce micro optimization

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Date:	Wed, 1 May 2013 13:00:47 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	kosaki.motohiro@...il.com
Cc:	linux-kernel@...r.kernel.org,
	Olivier Langlois <olivier@...llion01.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	David Miller <davem@...emloft.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Subject: Re: [PATCH 09/10] sched: task_sched_runtime introduce micro
 optimization

On Mon, Apr 29, 2013 at 11:17:17PM -0400, kosaki.motohiro@...il.com wrote:
> From: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
> 
> rq lock in task_sched_runtime() is necessary for two reasons. 1)
> accessing se.sum_exec_runtime is inatomic on 32bit and 2)
> do_task_delta_exec() require it.
> 
> And then, 64bit can avoid holds rq lock when add_delta is false.
> 
> Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
> ---
>  kernel/sched/core.c |    6 ++++++
>  1 files changed, 6 insertions(+), 0 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index b817e6d..24ba1c6 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2657,6 +2657,12 @@ unsigned long long task_sched_runtime(struct task_struct *p, bool add_delta)
>  	struct rq *rq;
>  	u64 ns = 0;
>  
> +	/* Micro optimization. */

Instead of the above; how about something like:

  /* 64-bit doesn't need locks to atomically read a 64bit value */

> +#ifdef CONFIG_64BIT
> +	if (!add_delta)
> +		return p->se.sum_exec_runtime;
> +#endif
> +
>  	rq = task_rq_lock(p, &flags);
>  	ns = p->se.sum_exec_runtime;
>  	if (add_delta)
> -- 
> 1.7.1
> 
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