linux-kernel - Re: [PATCH 2/6] nohz: dataplane: allow tick to be fully disabled for dataplane

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Date:	Tue, 12 May 2015 06:12:50 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Chris Metcalf <cmetcalf@...hip.com>,
	Gilad Ben Yossef <giladb@...hip.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>, Tejun Heo <tj@...nel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Christoph Lameter <cl@...ux.com>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/6] nohz: dataplane: allow tick to be fully disabled for
 dataplane

On Tue, May 12, 2015 at 11:26:07AM +0200, Peter Zijlstra wrote:
> On Fri, May 08, 2015 at 01:58:43PM -0400, Chris Metcalf wrote:
> > While the current fallback to 1-second tick is still helpful for
> > maintaining completely correct kernel semantics, processes using
> > prctl(PR_SET_DATAPLANE) semantics place a higher priority on running
> > completely tickless, so don't bound the time_delta for such processes.
> > 
> > This was previously discussed in
> > 
> > https://lkml.org/lkml/2014/10/31/364
> > 
> > and Thomas Gleixner observed that vruntime, load balancing data,
> > load accounting, and other things might be impacted.  Frederic
> > Weisbecker similarly observed that allowing the tick to be indefinitely
> > deferred just meant that no one would ever fix the underlying bugs.
> > However it's at least true that the mode proposed in this patch can
> > only be enabled on an isolcpus core, which may limit how important
> > it is to maintain scheduler data correctly, for example.
> 
> So how is making this available going to help people fix the actual
> problem?

It will at least provide an environment where adding more of this
problem might get punished.  This would be an improvement over what
we have today, namely that the 1HZ fallback timer silently forgives
adding more problems of this sort.

							Thanx, Paul

> There is nothing fundamentally impossible about fixing this proper, its
> just a lot of hard work.
> 
> NAK on this, do it right.
> 

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