linux-kernel - Re: [PATCH v4 1/5] nohz_full: add support for "cpu

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Message-ID: <20150722192834.GA32511@linux.vnet.ibm.com>
Date:	Wed, 22 Jul 2015 12:28:34 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Chris Metcalf <cmetcalf@...hip.com>,
	Gilad Ben Yossef <giladb@...hip.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	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>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	Linux API <linux-api@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 1/5] nohz_full: add support for "cpu_isolated" mode

On Wed, Jul 22, 2015 at 08:57:45AM -0500, Christoph Lameter wrote:
> On Tue, 21 Jul 2015, Paul E. McKenney wrote:
> 
> > > > In the current version, there's also the vmstat_update() that
> > > > may schedule delayed work and interrupt the core again
> > > > shortly before realizing that there are no more counter updates
> > > > happening, at which point it quiesces.  Currently we handle
> > > > this in cpu_isolated mode simply by spinning and waiting for
> > > > the timer interrupts to complete.
> > >
> > > Perhaps we should fix that?
> >
> > Didn't Christoph Lameter fix this?  Or is this an additional problem?
> 
> Well the vmstat update must realize first that there are no outstanding
> updates before switching itself off. So typically there is one extra tick.
> But we could add another function that will simply fold the differential
> immediately and turn the kworker task in the expectation that the
> processor will stay quiet.

Got it, thank you!

								Thanx, Paul

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