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Date: Wed, 16 Jan 2013 14:06:48 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
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        Paul Gortmaker <paul.gortmaker@windriver.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH v4] context_tracking: Add comments on interface and
 internals
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On Wed, 16 Jan 2013 16:49:21 -0500
Steven Rostedt <rostedt@goodmis.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.