Re: [PATCH 10/13] remove aggressive idle balancing

[Nick Piggin]

Siddha, Suresh B wrote:
> Nick,
>
> On Mon, Mar 07, 2005 at 07:28:23PM +1100, Nick Piggin wrote:
>
> > Siddha, Suresh B wrote:
> >
> > > We are resetting the nr_balance_failed to cache_nice_tries after kicking 
> > > active balancing. But can_migrate_task will succeed only if
> > > nr_balance_failed > cache_nice_tries.
> >
> > It is indeed, thanks for catching that. We should probably make it
> > reset the count to the point where it will start moving cache hot
> > tasks (ie. cache_nice_tries+1).
>
>
> That still might not be enough. We probably need to pass push_cpu's
> sd to move_tasks call in active_load_balance, instead of current busiest_cpu's
> sd. Just like push_cpu, we need to add one more field to the runqueue which 
> will specify the domain level of the push_cpu at which we have an imbalance.

It should be the lowest domain level that spans both this_cpu and
push_cpu, and has the SD_BALANCE flag set. We could possibly be a bit
more general here, but so long as nobody is coming up with weird and
wonderful sched_domains schemes, push_cpu should give you all the info
needed.

> > Ah yep, right you are there, too. I obviously hadn't looked closely
> > enough at the recent active_load_balance patches that had gone in :(
> > What should probably do is heed the "push_cpu" prescription (push_cpu
> > is now unused).
>
>
> push_cpu might not be the ideal destination in all cases. Take a NUMA domain
> above SMT+SMP domains in my above example. Assume P0, P1 is in node-0 and
> P2, P3 in node-1. Assume Loads of P0,P1,P2 are same as the above example,with P3
> containing one process load. Now any idle thread in P2 or P3 can trigger
> active load balance on P0. We should be selecting thread in P2 ideally
> (currently this is what we get with idle package check). But with push_cpu,
> we might move to the idle thread in P3 and then finally move to P2(it will be a
> two step process)

Hmm yeah. It is a bit tricky. We don't currently do exceptionally well
at this sort of "balancing over multiple domains" very well in the
periodic balancer either.

But at this stage I prefer to not get overly complex, and allow some
imperfect task movement, because it should rarely be a problem, and is
much better than it was before. The main place where it can go wrong
is multi-level NUMA balancing, where moving a task twice (between
different nodes) can cause more problems.

Nick

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