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 16:42:18 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Rakib Mullick <rakib.mullick@...il.com>
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 Thu, 2012-08-16 at 20:28 +0600, Rakib Mullick wrote:

> 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.

Sure but affinities and cpusets can still scatter, and therefore your
logic doesn't hold up, but see below.

> > 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.

It is not.. nr_uninterruptible is incremented on the cpu the task goes
to sleep and decremented on the cpu doing the wakeup.

This means that nr_uninterruptible is a complete mess and any per-cpu
value isn't meaningful at all.

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.
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