Re: allow the load to grow upto its cpu_power (was Re: [Patch] don't kick ALB in the presence of pinned task)

[Ingo Molnar]

* Siddha, Suresh B <suresh.b.siddha@xxxxxxxxx> wrote:

> On Tue, Aug 02, 2005 at 11:27:17AM +0200, Ingo Molnar wrote:
> > 
> > * Siddha, Suresh B <suresh.b.siddha@xxxxxxxxx> wrote:
> > 
> > > Jack Steiner brought this issue at my OLS talk.
> > > 
> > > Take a scenario where two tasks are pinned to two HT threads in a physical
> > > package. Idle packages in the system will keep kicking migration_thread
> > > on the busy package with out any success.
> > > 
> > > We will run into similar scenarios in the presence of CMP/NUMA.
> > > 
> > > Patch appended.
> > > 
> > > Signed-off-by: Suresh Siddha <suresh.b.siddha@xxxxxxxxx>
> > 
> > nice catch!
> > 
> > fine for -mm, but i dont think we need this fix in 2.6.13, as the effect 
> > of the bug is an extra context-switch per 'CPU goes idle' event, in this 
> > very specific (and arguably broken) task binding scenario.
> 
> No. This is not a broken scenario. Its possible in NUMA case aswell.
> 
> For example, lets take two nodes each having two physical packages. 
> And assume that there are two tasks and both of them are on (may or 
> may n't be pinned) two packages in node-0
> 
> Todays load balance will detect that there is an imbalance between the 
> two nodes and will try to distribute the load between the nodes.
> 
> In general, we should allow the load of a group to grow upto its 
> cpu_power and stop preventing these costly movements.
> 
> Appended patch will fix this. I have done limited testing of this 
> patch. Guys with big NUMA boxes, please give this patch a try.

makes sense in general - we should not try to move things around when we 
are under-utilized. (In theory there could be heavily fluctuating 
workloads which do not produce an above 100% average utilization, and 
which could benefit from a perfectly even distribution of tasks - but i 
dont think the load-balancer should care, as load-balancing is mostly a 
"slow" mechanism.)

Again, 2.6.14 stuff.

Acked-by: Ingo Molnar <mingo@xxxxxxx>

	Ingo
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