linux-kernel - Re: [PATCH] sched: Reduce the rate of needless idle load balancing

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Message-ID: <CAGQ1y=4z69iNRAM8eZFhh=kjRYQgwDxyMjyNG4XJ0b1xOxLURA@mail.gmail.com>
Date:	Tue, 20 May 2014 14:09:26 -0700
From:	Jason Low <jason.low2@...com>
To:	Tim Chen <tim.c.chen@...ux.intel.com>
Cc:	Jason Low <jason.low2@...com>, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Len Brown <len.brown@...el.com>, Russ Anderson <rja@....com>,
	Dimitri Sivanich <sivanich@....com>,
	Hedi Berriche <hedi@....com>, Andi Kleen <andi@...stfloor.org>,
	Michel Lespinasse <walken@...gle.com>,
	Rik van Riel <riel@...hat.com>,
	Peter Hurley <peter@...leysoftware.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched: Reduce the rate of needless idle load balancing

On Tue, May 20, 2014 at 1:59 PM, Tim Chen <tim.c.chen@...ux.intel.com> wrote:
> On Tue, 2014-05-20 at 13:51 -0700, Jason Low wrote:
>> On Tue, May 20, 2014 at 1:17 PM, Tim Chen <tim.c.chen@...ux.intel.com> wrote:
>>
>> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> > index 9b4c4f3..97132db 100644
>> > --- a/kernel/sched/fair.c
>> > +++ b/kernel/sched/fair.c
>> > @@ -6764,12 +6764,17 @@ static void nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
>> >
>> >                 rq = cpu_rq(balance_cpu);
>> >
>> > -               raw_spin_lock_irq(&rq->lock);
>> > -               update_rq_clock(rq);
>> > -               update_idle_cpu_load(rq);
>> > -               raw_spin_unlock_irq(&rq->lock);
>> > -
>> > -               rebalance_domains(rq, CPU_IDLE);
>> > +               /*
>> > +                * If time for next balance is due,
>> > +                * do the balance.
>> > +                */
>> > +               if (time_after(jiffies + 1, rq->next_balance)) {
>>
>> Hi Tim,
>>
>> If we want to do idle load balancing only when it is due for a
>> balance, shouldn't the above just be "if (time_after(jiffies,
>> rq->next_balance))"?
>
> If rq->next_balance and jiffies are equal, then
> time_after(jiffies, rq->next_balance) check will be false and
> you will not do balance.  But actually you want to balance
> for this case so the jiffies+1 was used.

Hi Tim, Rik

Yes, that makes sense that we want to balance if they are equal. We
may also consider using "if (time_after_eq(jiffies,
rq->next_balance)".

Reviewed-by: Jason Low <jason.low2@...com>
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