linux-kernel - Re: busted CFS group load balancer?

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Message-ID: <b040c32a0811180927r77e0a112s97a01d20d3cbe3d5@mail.gmail.com>
Date:	Tue, 18 Nov 2008 09:27:14 -0800
From:	Ken Chen <kenchen@...gle.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Chris Friesen <cfriesen@...tel.com>, Ingo Molnar <mingo@...e.hu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: busted CFS group load balancer?

On Tue, Nov 18, 2008 at 4:30 AM, Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
>> I'm also very troubled with this calculation in __update_group_shares_cpu():
>>
>>         shares = (sd_shares * rq_weight) / (sd_rq_weight + 1);
>>
>> Won't you have rounding problem here?  value 'shares' will gradually
>> decrease for each iteration of __update_group_shares_cpu()?
>
> Yes it will, however at the top of the sched-domain tree its reset.
>
>        if (!sd->parent || !(sd->parent->flags & SD_LOAD_BALANCE))
>                shares = tg->shares;

Hmm?  it is only true mostly on flat smp machine, or on numa system
where task is woken up across a sched-domain.

There is this code in try_to_wake_up():

                for_each_domain(this_cpu, sd) {
                        if (cpu_isset(cpu, sd->span)) {
                                update_shares(sd);
                                break;
                        }
                }

It doesn't iterate to the top half of the time on numa machine.

- Ken
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