Re: [PATCH] i386 No-Idle-Hz aka Dynamic-Ticks 3

[Tony Lindgren <tony@atomide.com>]

* Zwane Mwaikambo <zwane@arm.linux.org.uk> [050809 07:17]:
> On Mon, 8 Aug 2005, Tony Lindgren wrote:
> 
> > As far as I remember enabling AMD stop grant disconnects all cpus. This
> > means the system won't be able to do any work until the dyntick timer
> > interrupt wakes up the system.
> > 
> > > Both requirements (idling all CPUs together vs individually) I think
> > > will make the patch more complex.  Are such systems (which require having to 
> > > idle all CPUs together) pretty common that we have to care about?!
> > 
> > Probably all AMD SMP based systems? Somebody with better knowledge should
> > verify this.
> 
> It would be the K7 only.

OK, still quite a few systems.

> > > But that may be too late on some CPUs. If dyn_tick->skip = 100, all
> > > CPUs skip 100 ticks. However some CPUs may have timers that need to be
> > > service much before that.
> > 
> > Not in the current case, as the system is completely idle until some
> > interrupt wakes up the system. Of course it would be different if you make
> > it per-CPU.
> 
> I once did a weekend version of this with SMP support and for the PIT, i 
> had the last cpu to enter idle turn reprogram the PIT. Unfortunately this 
> means waiting for all processors and isn't as effective as a result.
> 
> > Well we need to be able to do various things in the idle loop depending on
> > the length of the estimated sleep. For example, if next_timer_interrupt is
> > 2 jiffies away, we cannot do much. But if next_timer_interrupt is 2 seconds
> > away, we can idle pretty much all devices.
> > 
> > > > But in any case on P4 systems the APIC timer is not the bottleneck as
> > > > stopping or reprogramming PIT also kills APIC. (This does not happen on P3
> > > > systems). So the bottleneck most likely is the length of PIT.
> > > 
> > > On these systems, do you disabled APIC (dyntick=noapic)?
> > 
> > Yeah. It only seems to work on P3 systems.
> 
> Odd, does reprogramming the APIC at that point get it going again?

Hmmm, might be worth trying.

Tony