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

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Date:	Mon, 28 Apr 2014 11:04:34 -0700
From:	Jason Low <jason.low2@...com>
To:	Preeti Murthy <preeti.lkml@...il.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.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>
Subject: Re: [PATCH 1/3] sched, balancing: Update rq->max_idle_balance_cost
 whenever newidle balance is attempted

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).

> 
> Besides this:
> Reviewed-by: Preeti U Murthy <preeti@...ux.vnet.ibm.com>

Thanks for the review.

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