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

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Date:	Sun, 25 Aug 2013 21:36:48 -0700
From:	Paul Turner <pjt@...gle.com>
To:	Lei Wen <adrian.wenl@...il.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 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.
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