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SubjectRe: 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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