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

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Date:	Tue, 29 Apr 2014 08:40:04 +0530
From:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Preeti Murthy <preeti.lkml@...il.com>,
	Jason Low <jason.low2@...com>, 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>, aswin@...com,
	chegu_vinod@...com
Subject: Re: [PATCH 1/3] sched, balancing: Update rq->max_idle_balance_cost
 whenever newidle balance is attempted

On 04/28/2014 02:54 PM, Peter Zijlstra wrote:
> On Sun, Apr 27, 2014 at 02:01:45PM +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;
> 
> So the reason I didn't do that is that nothing else does that either.
> Also, note that the value we set rq->next_balance to might itself
> already be expired. There is no guarantee that last_balance + interval
> is in the future.
> 
Hmm this makes sense. Thanks!

Regards
Preeti U Murthy

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