Re: [CPUISOL] CPU isolation extensions

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Mon, 2008-01-28 at 11:34 -0500, Steven Rostedt wrote:
> On Mon, Jan 28, 2008 at 08:59:10AM -0600, Paul Jackson wrote:
> > Thanks for the CC, Peter.
> 
> Thanks from me too.
> 
> > Max wrote:
> > > We've had scheduler support for CPU isolation ever since O(1) scheduler went it. 
> > > I'd like to extend it further to avoid kernel activity on those CPUs as much as possible.
> > 
> > I recently added the per-cpuset flag 'sched_load_balance' for some
> > other realtime folks, so that they can disable the kernel scheduler
> > load balancing on isolated CPUs.  It essentially allows for dynamic
> > control of which CPUs are isolated by the scheduler, using the cpuset
> > hierarchy, rather than enhancing the 'isolated_cpus' mask.   That
> > 'isolated_cpus' mask remained a minimal kernel boottime parameter.
> > I believe this went to Linus's tree about Oct 2007.
> > 
> > It looks like you have three additional tweaks for realtime in this
> > patch set, with your patches:
> > 
> >   [PATCH] [CPUISOL] Do not route IRQs to the CPUs isolated at boot
> 
> I didn't know we still routed IRQs to isolated CPUs. I guess I need to
> look deeper into the code on this one. But I agree that isolated CPUs
> should not have IRQs routed to them.

While I agree with this in principle, I'm not sure flat out denying all
IRQs to these cpus is a good option. What about the case where we want
to service just this one specific IRQ on this CPU and no others?

Can't this be done by userspace irq routing as used by irqbalanced?

> >   [PATCH] [CPUISOL] Support for workqueue isolation
> 
> The thing about workqueues is that they should only be woken on a CPU if
> something on that CPU accessed them. IOW, the workqueue on a CPU handles
> work that was called by something on that CPU. Which means that
> something that high prio task did triggered a workqueue to do some work.
> But this can also be triggered by interrupts, so by keeping interrupts
> off the CPU no workqueue should be activated.

Quite so, if nobody uses it, there is no harm in having them around. If
they are used, its by someone already allowed on the cpu.

> >   [PATCH] [CPUISOL] Isolated CPUs should be ignored by the "stop machine"
> 
> This I find very dangerous. We are making an assumption that tasks on an
> isolated CPU wont be doing things that stopmachine requires. What stops
> a task on an isolated CPU from calling something into the kernel that
> stop_machine requires to halt?

Very dangerous indeed!