linux-kernel - Re: false nr_running check in load balance?

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Message-ID: <CAPM31R+zi4yO8N-bhHWfmPsSGOV7_tMPrYoJ4q5AqVRA4tBNeQ@mail.gmail.com>
Date:	Tue, 13 Aug 2013 02:25:46 -0700
From:	Paul Turner <pjt@...gle.com>
To:	Lei Wen <adrian.wenl@...il.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>, Lei Wen <leiwen@...vell.com>
Subject: Re: false nr_running check in load balance?

On Tue, Aug 13, 2013 at 1:18 AM, Lei Wen <adrian.wenl@...il.com> wrote:
> Hi Paul,
>
> On Tue, Aug 13, 2013 at 4:08 PM, Paul Turner <pjt@...gle.com> wrote:
>> On Tue, Aug 13, 2013 at 12:38 AM, Peter Zijlstra <peterz@...radead.org> wrote:
>>> On Tue, Aug 13, 2013 at 12:45:12PM +0800, Lei Wen wrote:
>>>> > Not quite right; I think you need busiest->cfs.h_nr_running.
>>>> > cfs.nr_running is the number of entries running in this 'group'. If
>>>> > you've got nested groups like:
>>>> >
>>>> >  'root'
>>>> >    \
>>>> >    'A'
>>>> >    / \
>>>> >   t1 t2
>>>> >
>>>> > root.nr_running := 1 'A', even though you've got multiple running tasks.
>>>> >
>>>>
>>>> You're absolutely right for this. :)
>>>> I miss it for not considering the group case...
>>>>
>>>> Then do you think it is necessary to do below change in load_balance() code?
>>>>  -       if (busiest->nr_running > 1) {
>>>>  +       if (busiest->cfs.h_nr_running > 1) {
>>>>
>>>
>>> Yes I think that would be fine.
>>
>> If we pivot to use h_nr_running we should probably also update
>> call-sites such as cpu_load_avg_per_task() for consistency.
>
> I didn't find cpu_load_avg_per_task in the latest linux git...
> Is it a new patch pending while not being submitted?

Transposition typo: cpu_avg_load_per_task()
More generally: Most things that examine ->nr_running in the fair
load-balance path.

>
> Thanks,
> Lei
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