linux-kernel - Re: nohz: Warn if the machine can not perform nohz

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Date:	Mon, 15 Jul 2013 13:24:23 -0400
From:	Dave Jones <davej@...hat.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	rostedt@...dmis.org
Subject: Re: nohz: Warn if the machine can not perform nohz_full

On Mon, Jul 15, 2013 at 07:18:02PM +0200, Frederic Weisbecker wrote:
 
 > > So I guess you guys never want this to be enabled on distro kernels ?
 > > If that's the case, can you add something to that effect in Kconfig ?
 > 
 > I believe we want it to be enabled on distros in the long term. But right now it would
 > be a bad idea until the off case (nohz_full= parameter empty) is carefully optimized.
 > I'm currently working on that.
 > 
 > Now for the unstable tsc, which is what it's about on the above code block, we need
 > the tick to be there to leverage the sched clock madness. May be there could be some
 > other solution that could work along full dynticks but for now we chose the easy path.
 > 
 > Are broken TSCs that common?
 
I just hit one apparently.  http://paste.fedoraproject.org/25421/73907845/raw/
That's a fairly recent Atom board, so I suspect it's not uncommon on that platform.

 > Also what is the preffered way to tell the distros that they shouldn't enable that option
 > for now? Here is what we currently have in the tail of the related Kconfig help:
 > 
 >         This is implemented at the expense of some overhead in user <-> kernel
 >         transitions: syscalls, exceptions and interrupts. Even when it's
 >         dynamically off.

"This feature is not ready to be deployed" ?

"This will taint the kernel if it decides it can't work" ?

	Dave

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