Messages in this thread |  | | | Date | Mon, 23 Feb 2009 10:11:58 +0100 | | From | Ingo Molnar <> | | Subject | Re: [RFC PATCH 0/4] timers: framework for migration between CPU |
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* 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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