linux-kernel - Re: [PATCH 03/10] sched: Clean-up struct sd_lb

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Date:	Mon, 26 Aug 2013 16:42:58 +0800
From:	Lei Wen <adrian.wenl@...il.com>
To:	Paul Turner <pjt@...gle.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	LKML <linux-kernel@...r.kernel.org>,
	Mike Galbraith <efault@....de>, Alex Shi <alex.shi@...el.com>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Namhyung Kim <namhyung@...nel.org>,
	Lei Wen <leiwen@...vell.com>, Rik van Riel <riel@...riel.com>,
	Joonsoo Kim <js1304@...il.com>
Subject: Re: [PATCH 03/10] sched: Clean-up struct sd_lb_stat

On Mon, Aug 26, 2013 at 12:36 PM, Paul Turner <pjt@...gle.com> wrote:
> On Sun, Aug 25, 2013 at 7:56 PM, Lei Wen <adrian.wenl@...il.com> wrote:
>> On Tue, Aug 20, 2013 at 12:01 AM, Peter Zijlstra <peterz@...radead.org> wrote:
>>> From: Joonsoo Kim <iamjoonsoo.kim@....com>
>>>
>>> There is no reason to maintain separate variables for this_group
>>> and busiest_group in sd_lb_stat, except saving some space.
>>> But this structure is always allocated in stack, so this saving
>>> isn't really benificial [peterz: reducing stack space is good; in this
>>> case readability increases enough that I think its still beneficial]
>>>
>>> This patch unify these variables, so IMO, readability may be improved.
>>>
>>> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@....com>
>>> [peterz: lots of style edits, a few fixes and a rename]
>>> Signed-off-by: Peter Zijlstra <peterz@...radead.org>
>>> Link: http://lkml.kernel.org/r/1375778203-31343-4-git-send-email-iamjoonsoo.kim@lge.com
>>> ---
>>>  kernel/sched/fair.c |  225 +++++++++++++++++++++++++---------------------------
>>>  1 file changed, 112 insertions(+), 113 deletions(-)
>>>
>>> --- a/kernel/sched/fair.c
>>> +++ b/kernel/sched/fair.c
>>> @@ -4277,36 +4277,6 @@ static unsigned long task_h_load(struct
>>>
>> [snip]...
>>> -       env->imbalance = DIV_ROUND_CLOSEST(
>>> -               sds->max_load * sds->busiest->sgp->power, SCHED_POWER_SCALE);
>>> +       env->imbalance = DIV_ROUND_CLOSEST(sds->busiest_stat.avg_load *
>>> +                               sds->busiest->sgp->power, SCHED_POWER_SCALE);
>>>
>>
>> I am wondering whether we could change this line as below is more appropriate,
>> since it would avoid the division here:
>>        env->imbalance = (sds->busiest_stat.avg_load * sds->busiest->sgp->power)
>>                                       >> SCHED_POWER_SHIFT;
>>
>> I am not sure whether compiler would be smarter enough to covert into
>>>> operation,
>> if it see SCHED_POWER_SCALE is 1024 here.
>
> This would change the rounding.  Fortunately, gcc is smart enough to
> handle this.

Indeed, I check the assembly code, and it is really smart to do the changes.

Thanks,
Lei
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