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 20:28:58 +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 19:45 +0600, Rakib Mullick wrote:
>> When a CPU is about to go down, it moves all it's sleeping task to an
>> active CPU, then nr_uninterruptible counts are
>> also moved. When moving nr_uninterruptible count, currently it chooses a
>> randomly picked CPU from the active CPU mask
>> to keep the global nr_uninterruptible count intact. But, it would be
>> precise to move nr_uninterruptible counts to the
>> CPU where all the sleeping tasks were moved and it also might have subtle
>> impact over rq's load calculation. So, this
>> patch is prepared to address this issue.
>
> The Changelog is ill-formated. Other than that, the patch doesn't appear
> to actually do what it says. The sleeping tasks can be scattered to any
> number of cpus as decided by select_fallback_rq().
>
I'm not sure which parts are missing from Changelog to patch. And this
patch assumes that, sleeping tasks won't be scattered. From
select_fallback_rq(), sleeping tasks might get scattered due to
various cases like. if CPU is down, task isn't allowed to move a
particular CPU. Other than that, dest cpu supposed to be the same.

> Furthermore there should be absolutely no impact on load calculation
> what so ever. nr_uninterruptible is only ever useful as a sum over all
> cpus, this total sum doesn't change regardless of where you put the
> value.
>
> Worse, there's absolutely no relation to the tasks on the runqueue
> (sleeping or otherwise) and nr_uninterruptible, so coupling these
> actions makes no sense what so ever.
>
nr_uninterruptible is coupled with tasks on the runqueue to calculate
nr_active numbers.
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.

Thanks,
Rakib
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