linux-kernel - Re: sched: Improve load balancing in the presence of idle CPUs

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Date:	Tue, 07 Apr 2015 10:42:41 -0700
From:	Jason Low <jason.low2@...com>
To:	Tim Chen <tim.c.chen@...ux.intel.com>
Cc:	Morten Rasmussen <morten.rasmussen@....com>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	"peterz@...radead.org" <peterz@...radead.org>,
	"mingo@...nel.org" <mingo@...nel.org>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	"riel@...hat.com" <riel@...hat.com>,
	"vincent.guittot@...aro.org" <vincent.guittot@...aro.org>,
	"srikar@...ux.vnet.ibm.com" <srikar@...ux.vnet.ibm.com>,
	"pjt@...gle.com" <pjt@...gle.com>,
	"benh@...nel.crashing.org" <benh@...nel.crashing.org>,
	"efault@....de" <efault@....de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"iamjoonsoo.kim@....com" <iamjoonsoo.kim@....com>,
	"svaidy@...ux.vnet.ibm.com" <svaidy@...ux.vnet.ibm.com>,
	jason.low2@...com
Subject: Re: sched: Improve load balancing in the presence of idle CPUs

On Fri, 2015-04-03 at 15:35 -0700, Tim Chen wrote:
> I think we can get rid of the done_balancing boolean 
> and make it a bit easier to read if we change the above code to
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index bcfe320..08317dc 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7557,8 +7557,13 @@ static void nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
>                  * work being done for other cpus. Next load
>                  * balancing owner will pick it up.
>                  */
> -               if (need_resched())
> -                       break;
> +               if (need_resched()) {
> +                       /* preparing to bail, kicking other cpu to continue */
> +                       clear_bit(NOHZ_BALANCE_KICK, nohz_flags(this_cpu));
> +                       if (nohz_kick_needed(this_rq))
> +                               nohz_balance_kick();
> +                       return;
> +               }

Hi Tim,

We would also need the nohz_kick_needed/nohz_balance_kick if we
initially find that the current CPU is not idle (at the beginning of
nohz_idle_balance). In the above case, we would need to add the code to
2 locations.

Would it be better to still keep the done_balancing to avoid having
duplicate code?

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