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

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Date:	Wed, 01 May 2013 14:42:28 -0400
From:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	kosaki.motohiro@...il.com, 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

(5/1/13 7:00 AM), Peter Zijlstra wrote:
> 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 */

Looks nicer. Indeed.



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