Re: + stupid-hack-to-make-mainline-build.patch added to -mm tree

Thomas Gleixner
2007-03-07T01:16:47Z

On Tue, 2007-03-06 at 16:42 -0800, Dan Hecht wrote:
> >> accounting would be wrong. Instead, we should allow the
> >> tick_sched_timer in cases (c) and (d) to have runtime configurable
> >> period, and then scale the time value accordingly before passing to
> >> account_system_time. This is probably something the Xen folks will want
> >> also, since I think Xen itself only gets 100hz hard timer, and so it can
> >> implement at best a oneshot virtual timer with 100hz resolution. Any
> >> objections to us doing something like this?
> >
> > Yes. It's gross hackery.
> >
> > 1) We want to have a cleanup of the tick assumptions _all_ over the
> > place and this is going to be real hard work.
> >
> > 2) As I said above. The time accounting for virtualization needs to be
> > fixed in a generic way.
> >
> > I'm not going to accept some weird hackery for virtualization, which is
> > of exactly ZERO value for the kernel itself. Quite the contrary it will
> > make the cleanup harder and introduce another hard to remove thing,
> > which will in the worst case last for ever.
> >
>
> Okay, to confirm I'm on the same page as you, you want to move process
> time accounting from being periodic sampled based to being trace based?
> i.e. at the system-call/interrupt boundaries, read clocksource and
> compute directly the amount of system/user/process time?

At least for the paravirt guests this is the correct approach. Once the
CPU vendors come up with a sane solution for a reliable and fast clock
source we might use that on real hardware as well.

> Do you know if anyone has explored this? I thought there was a
> discussion about this a while back but it was rejected due to the
> sample-based approach having much lower overheads on high system call
> rate workloads.

Yes, with todays hardware it is simply a PITA. PowerPC has some basic
support for this though, IIRC.

tglx