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SubjectRe: [RFC PATCH 0/4] timers: framework for migration between CPU

* Balbir Singh <balbir@linux.vnet.ibm.com> wrote:

> * Ingo Molnar <mingo@elte.hu> [2009-02-20 22:53:18]:
>
> >
> > * Arjan van de Ven <arjan@infradead.org> wrote:
> >
> > > On Fri, 20 Feb 2009 17:07:37 +0100
> > > Ingo Molnar <mingo@elte.hu> wrote:
> > >
> > > >
> > > > * Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com> wrote:
> > > >
> > > > > > I'd also suggest to not do that rather ugly
> > > > > > enable_timer_migration per-cpu variable, but simply reuse
> > > > > > the existing nohz.load_balancer as a target CPU.
> > > > >
> > > > > This is a good idea to automatically bias the timers. But
> > > > > this nohz.load_balancer is a very fast moving target and we
> > > > > will need some heuristics to estimate overall system idleness
> > > > > before moving the timers.
> > > > >
> > > > > I would agree that the power saving load balancer has a good
> > > > > view of the system and can potentially guide the timer biasing
> > > > > framework.
> > > >
> > > > Yeah, it's a fast moving target, but it already concentrates
> > > > the load somewhat.
> > > >
> > >
> > > I wonder if the real answer for this isn't to have timers be
> > > considered schedulable-entities and have the regular scheduler
> > > decide where they actually run.
> >
> > hm, not sure - it's a bit heavy for that.
> >
>
> I think the basic timer migration policy should exist in user
> space.

I disagree.

> One of the ways of looking at it is, as we begin to
> consolidate, using range timers and migrating all timers to
> lesser number of CPUs would make a whole lot of sense.
>
> As far as the scheduler making those decisions is concerned,
> my concern is that the load balancing is a continuous process
> and timers don't necessarily work that way. I'd put my neck
> out and say that irqbalance, range timers and timer migration
> should all belong to user space. irqbalance and range timers
> do, so should timer migration.

As i said it my first reply, IRQ migration is special because
they are not kernel-internal objects, they come externally so
there's a lot of user-space enumeration, policy and other steps
involved. Furthermore, IRQs are migrated in a 'slow' fashion.

Timers on the other hand are fast entities tied to _tasks_
primarily, not external entities. Hence they should migrate
according to the CPU where the activities of the system
concentrates - i.e. where tasks are running.

Another thing: do you argue for the existing timer-migration
code we have in mod_timer() to move to user-space too? It isnt a
consistent argument to push 'some' of it to user-space, and some
of it in kernel-space.

Ingo


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