linux-kernel - Re: Add rq->nr_uninterruptible count to dest cpu's rq while CPU goes down.

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Date:	Thu, 16 Aug 2012 21:32:45 +0600
From:	Rakib Mullick <rakib.mullick@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	mingo@...nel.org, linux-kernel@...r.kernel.org,
	paulmck <paulmck@...ux.vnet.ibm.com>
Subject: Re: Add rq->nr_uninterruptible count to dest cpu's rq while CPU goes down.

On 8/16/12, Peter Zijlstra <peterz@...radead.org> wrote:
> On Thu, 2012-08-16 at 20:28 +0600, Rakib Mullick wrote:
>
>> nr_uninterruptible is coupled with tasks on the runqueue to calculate
>> nr_active numbers.
>
> It is not.. nr_uninterruptible is incremented on the cpu the task goes
> to sleep and decremented on the cpu doing the wakeup.
>
If nr_uninterruptible's life cycle is this simple then,  while CPU
goes down, nr_uninterruptible count will be decremented when all the
tasks are moved to other CPUs and should be fine.

> This means that nr_uninterruptible is a complete mess and any per-cpu
> value isn't meaningful at all.
>
Well, if nr_uninterruptible is a mess, then this patch has no meaning.
And also I think migrate_nr_uninterruptible() is meaning less too.

> It is quite possible to always have the inc on cpu0 and the decrement on
> cpu1, yielding results like:
>
> {1000, -1000} for an effective nr_uninterruptible = 0. Taking either cpu
> down will then migrate whatever delta it has to another cpu, but there
> might only be a single task, yet the delta is +-1000.
>
>> In calc_load_fold_active(), this nr_active numbers are used to
>> calculate delta. This is how I understand this part and seeing some
>> impact.
>
> You understand wrong, please re-read the comment added in commit
> 5167e8d5.
>
Yes, reading.

Thanks,
Rakib.
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