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

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Message-ID: <20130716010135.GB13562@somewhere>
Date:	Tue, 16 Jul 2013 03:01:36 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Dave Jones <davej@...hat.com>,
	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 01:24:23PM -0400, Dave Jones wrote:
> 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" ?

I can try this one. Or may be I should be more direct and put:

"This feature is not ready to be deployed on distros"

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