linux-kernel - Re: [RFC PATCH 4/4] sched: Upload nohz full CPU load on task enqueue/dequeue

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Date:	Wed, 20 Jan 2016 16:11:14 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Frederic Weisbecker <fweisbec@...il.com>
cc:	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Byungchul Park <byungchul.park@....com>,
	Chris Metcalf <cmetcalf@...hip.com>,
	Luiz Capitulino <lcapitulino@...hat.com>,
	Christoph Lameter <cl@...ux.com>,
	"Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
	Mike Galbraith <efault@....de>, Rik van Riel <riel@...hat.com>
Subject: Re: [RFC PATCH 4/4] sched: Upload nohz full CPU load on task
 enqueue/dequeue

On Wed, 20 Jan 2016, Frederic Weisbecker wrote:
> On Wed, Jan 20, 2016 at 10:09:06AM +0100, Peter Zijlstra wrote:
> > Also, since when can we have enqueues/dequeues while NOHZ_FULL ? I
> > thought that was the 1 task 100% cpu case, there are no
> > enqueues/dequeues there.
> 
> That's the most optimized case but we can definetly have small moments with
> more than one task running. For example if we have a workqueue, or such
> short and quick tasks.
>
> If the user makes use of full dynticks for soft isolation (for performance,
> can live with a few interrupts...), there can be short moments of
> multitasking.

Again, you are trying to make the second step after the first one is
completed. We do not even have proper accounting when we have the ONE task
100% case and still you try to solve problems beyond that.

If that ONE task gets interrupted, then accounting should take place.

When there is another runnable task then that nohz state needs to be left. You
can go back to it once the task is alone again.

You are trying to make the complete accounting 'almost' tick independent, but
approaching that from the tick nohz angle is wrong.

When you really want to go there, and I can see why you want that, then you
need to solve this from ground up and such a solution has nothing to do with
any flavour of NOHZ. That simply needs to rework the whole accounting
machinery and rip out the complete tick dependency. Once you have that your
NOHZ business falls into place. Any other approach is just duct tape
engineering.

Thanks,

	tglx