linux-kernel - Re: [PATCH 1/3] sched, balancing: Update rq->max_idle_balance_cost whenever newidle balance is attempted

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Message-ID: <535F21E0.9020305@linux.vnet.ibm.com>
Date:	Tue, 29 Apr 2014 09:22:00 +0530
From:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
To:	Jason Low <jason.low2@...com>
CC:	Preeti Murthy <preeti.lkml@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Alex Shi <alex.shi@...aro.org>, Mike Galbraith <efault@....de>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Morten Rasmussen <morten.rasmussen@....com>
Subject: Re: [PATCH 1/3] sched, balancing: Update rq->max_idle_balance_cost
 whenever newidle balance is attempted

On 04/28/2014 11:34 PM, Jason Low wrote:
> On Sun, 2014-04-27 at 14:01 +0530, Preeti Murthy wrote:
>> Hi Jason, Peter,
>>
>> The below patch looks good to me except for one point.
>>
>> In idle_balance() the below code snippet does not look right:
>>
>> - if (pulled_task || time_after(jiffies, this_rq->next_balance)) {
>> - /*
>> - * We are going idle. next_balance may be set based on
>> - * a busy processor. So reset next_balance.
>> - */
>> +out:
>> + /* Move the next balance forward */
>> + if (time_after(this_rq->next_balance, next_balance))
>>   this_rq->next_balance = next_balance;
>> - }
>>
>> By not checking this_rq->next_balance against jiffies,
>> we might end up not updating this parameter when it
>> has expired.
>>
>> So shouldn't it be:
>>
>> if (time_after(jiffies, this_rq->next_balance) ||
>>            time_after(this_rq->next_balance, next_balance))
>>     this_rq->next_balance = next_balance;
> 
> Hi Preeti,
> 
> If jiffies is after this_rq->next_balance, doesn't that mean that it's
> actually due for a periodic balance and we wouldn't need to modify it?
> In rebalance_domains(), we do load_balance if time_after_eq(jiffies,
> sd->last_balance + interval).

Right. So I missed the point that we don't really have a problem with
the rq->next_balance being expired. It will anyway ensure that in the
next call to rebalance_domains() load balancing will be done and that is
all we want. Thanks for pointing it out.

Regards
Preeti U Murthy
> 
>>
>> Besides this:
>> Reviewed-by: Preeti U Murthy <preeti@...ux.vnet.ibm.com>
> 
> Thanks for the review.
> 

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