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 17:45:31 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Thomas Gleixner <tglx@...utronix.de>
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, Jan 20, 2016 at 04:11:14PM +0100, Thomas Gleixner wrote:
> 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.

Alright, let me respawn that series with handling the most simple and common
scenario, which is 100% single task in full dynticks, and anything else keeps
the tick. That will indeed allow us a more incremental approach.

Thanks.