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

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Date:	Wed, 16 Jan 2013 16:49:21 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Andrew Morton <akpm@...ux-foundation.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, 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.

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?

-- Steve

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