linux-kernel - Re: [PATCH] nohz1: Documentation

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Date:	Thu, 21 Mar 2013 16:08:08 +0000
From:	Christoph Lameter <cl@...ux.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
cc:	Steven Rostedt <rostedt@...dmis.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Rob Landley <rob@...dley.net>, linux-kernel@...r.kernel.org,
	josh@...htriplett.org, zhong@...ux.vnet.ibm.com,
	khilman@...aro.org, geoff@...radead.org, tglx@...utronix.de
Subject: Re: [PATCH] nohz1: Documentation

On Wed, 20 Mar 2013, Paul E. McKenney wrote:

> > > Another approach is to offload RCU callback processing to "rcuo" kthreads
> > > using the CONFIG_RCU_NOCB_CPU=y.  The specific CPUs to offload may be
> > > selected via several methods:

Why are there multiple rcuo threads? Would a single thread that may be
able to run on multiple cpus not be sufficient?

> > "Even though the SCHED_FIFO task is the only task running, because the
> > SCHED_OTHER tasks are queued on the CPU, it currently will not enter
> > adaptive tick mode."
>
> Again, good point!

Uggh. That will cause problems and did cause problems when I tried to use
nohz.

The OS always has some sched other tasks around that become runnable after
a while (like for example the vm statistics update, or the notorious slab
scanning). As long as SCHED_FIFO is active and there is no process in the
same scheduling class then tick needs to be off. Also wish that this would
work with SCHED_OTHER if there is only a single task with a certain renice
value (-10?) and the rest is runnable at lower priorities. Maybe in that
case stop the tick for a longer period and then give the lower priority
tasks a chance to run but then switch off the tick again.
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