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SubjectRe: CFS review

* Roman Zippel <zippel@linux-m68k.org> wrote:

> > jiffies based sched_clock should be soon very rare. It's probably
> > not worth optimizing for it.
>
> I'm not so sure about that. sched_clock() has to be fast, so many
> archs may want to continue to use jiffies. [...]

i think Andi was talking about the vast majority of the systems out
there. For example, check out the arch demography of current Fedora
installs (according to the Smolt opt-in UUID based user metrics):

http://smolt.fedoraproject.org/

i686: 74743
x86_64: 18599
i386: 1208
ppc: 527
ppc64: 396
sparc64: 14
---------------
Total: 95488

even pure i386 (kernels, not systems) is a only 1.2% of all installs. By
the time the CFS kernel gets into a distro (a few months at minimum,
typically a year) this percentage will go down further. And embedded
doesnt really care about task-statistics corner cases [ (it likely
doesnt have 'top' installed - likely doesnt even have /proc mounted or
even built in ;-) ].

of course CFS should not do _worse_ stats than what we had before, and
should not break or massively misbehave. Also, anything sane we can do
for low-resolution arches we should do (and we already do quite a bit -
the while wmult stuff is to avoid expensive divisions) - and i regularly
booted CFS with a low-resolution clock to make sure it works. So i'm not
trying to duck anything, we've just got to keep our design priorities
right :-)

Ingo
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