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 17:52:23 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	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>,
	"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 09:19:36AM -0600, Christoph Lameter wrote:
> On Wed, 20 Jan 2016, Thomas Gleixner wrote:
> 
> > > 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.
> 
> "Soft" isolation? Like soft realtime ... Argh... Please stay away from
> corrupting the intents of the nohz full mode.

Well some people, especially real time, want no disturbance at all because
their workload really depends on that.

Some other (HPC) don't care about little disturbance.

Although HPC users didn't declare themselves yet, the real time case
expect some more sacrifices (see nohz tasks patchset).

> 
> > 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.
> 
> Please lets only deal with the one task issue. I can have "soft" isolation
> today by moving daemons etc off certain processors. We want definitely to
> run nothing else on the processor and will want to even go further with
> the cache allocation techniques in recent processor to limit the cache
> disturbances from other processors etc etc.

One task is common to all usecases anyway.