linux-kernel - Re: [PATCH v4] context_tracking: Add comments on interface and internals

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Date:	Wed, 16 Jan 2013 14:06:48 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Alessio Igor Bogani <abogani@...nel.org>,
	Chris Metcalf <cmetcalf@...era.com>,
	Christoph Lameter <cl@...ux.com>,
	Geoff Levand <geoff@...radead.org>,
	Gilad Ben Yossef <gilad@...yossef.com>,
	Hakan Akkan <hakanakkan@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	Li Zhong <zhong@...ux.vnet.ibm.com>,
	Namhyung Kim <namhyung.kim@....com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v4] context_tracking: Add comments on interface and
 internals

On Wed, 16 Jan 2013 16:49:21 -0500
Steven Rostedt <rostedt@...dmis.org> wrote:

> On Wed, 2013-01-16 at 13:18 -0800, Andrew Morton wrote:
> > On Wed, 16 Jan 2013 17:16:37 +0100
> 
> > I still don't understand NOHZ's role in this whole thing :(
> 
> It's not for idle NOHZ, but "process" NOHZ.
> 
> That is if we have only one task running on a CPU, we don't want a tick
> interrupt to bother it. This is because there's lots of users out there
> that want an uninterrupted task. A task that doesn't ever get bothered
> by the kernel. If it's in userspace, it stays in userspace (no
> interrupts), until it calls into the kernel itself (syscall). Even when
> its in the kernel, we still don't need the tick interrupt if its the
> only task.

oh, is that what TIF_NOHZ does ;)

> But the scheduler isn't the only thing that uses this tick. To remove
> the tick, we need to satisfy all the other users (printk, delayed work,
> u/s-times).
> 
> To still keep up the stats of user and kernel times for the task, we
> need to record when the task switches from user to kernel and back
> again. Hence the context tracking code.
> 
> Makes more sense?

yup thanks.
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