linux-kernel - Re: [PATCH 0/6] support "dataplane" mode for nohz

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Date:	Mon, 11 May 2015 17:36:04 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Chris Metcalf <cmetcalf@...hip.com>,
	Gilad Ben Yossef <giladb@...hip.com>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Rik van Riel <riel@...hat.com>, Tejun Heo <tj@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Christoph Lameter <cl@...ux.com>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	linux-doc@...r.kernel.org, linux-api@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/6] support "dataplane" mode for nohz_full

On Mon, May 11, 2015 at 08:57:59AM -0400, Steven Rostedt wrote:
> 
> NO_HZ_LEAVE_ME_THE_FSCK_ALONE!
> 
> 
> On Sat, 9 May 2015 09:05:38 +0200
> Ingo Molnar <mingo@...nel.org> wrote:
>  
> > So I think we should either rename NO_HZ_FULL to NO_HZ_PURE, or keep 
> > it at NO_HZ_FULL: because the intention of NO_HZ_FULL was always to be 
> > such a 'zero overhead' mode of operation, where if user-space runs, it 
> > won't get interrupted in any way.
> 
> 
> All kidding aside, I think this is the real answer. We don't need a new
> NO_HZ, we need to make NO_HZ_FULL work. Right now it doesn't do exactly
> what it was created to do. That should be fixed.
> 
> Please lets get NO_HZ_FULL up to par. That should be the main focus.

Now if we can achieve to make NO_HZ_FULL behave in a specific way
that fits everyone's usecase, I'll be happy.

But some people may expect hard isolation requirement (Real Time, deterministic
latency) and others softer isolation (HPC, only interested in performance, can
live with one rare random tick, so no need to loop before returning to userspace
until we have the no-noise guarantee).

I expect some Real Time users may want this kind of dataplane mode where a syscall
or whatever sleeps until the system is ready to provide the guarantee that no
disturbance is going to happen for a given time. I'm not sure HPC users are interested
in that.

In fact it goes along the fact that NO_HZ_FULL was really only supposed to be about
the tick and now people are introducing more and more kernel default presetting that
assume NO_HZ_FULL implies ISOLATION which is about all kind of noise (tick, tasks, irqs,
...). Which is true but what kind of ISOLATION?

Probably NO_HZ_FULL should really only be about stopping the tick then some sort
of CONFIG_ISOLATION would drive the kind of isolation we are interested in
and hereby the behaviour of NO_HZ_FULL, workqueues, timers, tasks affinity, irqs
affinity, dataplane mode, ...
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