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

[Ingo Molnar <mingo@elte.hu>]

* 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