Re: [patch] smpnice: don't consider sched groups which are lightly loaded for balancing

[Peter Williams <pwil3058@bigpond.net.au>]

Andrew Morton wrote:
> "Siddha, Suresh B" <suresh.b.siddha@intel.com> wrote:
>> updated patch appended. thanks.
> 
> Where are we up to with smpnice now?  Are there still any known
> regressions/problems/bugs/etc?

One more change to move_tasks() is required to address an issue raised 
by Suresh w.r.t. the possibility unnecessary movement of the highest 
priority task from the busiest queue (possible because of the 
active/expired array mechanism).  I hope to forward a patch for this 
later today.

After that the only thing I would like to do at this stage is modify 
try_to_wake_up() so that it tries harder to distribute high priority 
tasks across the CPUs.  I wouldn't classify this as absolutely necessary 
as it's really just a measure that attempts to reduce latency for high 
priority tasks as it should get them onto a CPU more quickly than just 
sticking them anywhere and waiting for load balancing to kick in if 
they've been put on a CPU with a higher priority task already running. 
Also it's only really necessary when there a lot of high priority tasks 
running.  So this isn't urgent and probably needs to be coordinated with 
Ingo's RT load balancing stuff anyway.

>  Has sufficient testing been done for us to
> know this?

I run smpnice kernels on all of my SMP machines all of the time.  But I 
don't have anything with more than 2 CPUs so I've been relying on their 
presence in -mm to get wider testing on larger machines.

I think that once this patch and the move_tasks() one that I'll forward 
later today are incorporated we should have something that (although not 
perfect) works pretty well.  Neither of these changes should cause a 
behavioural change in the case where all tasks are nice==0.

As load balancing is inherently probabilistic I don't think that we 
should hold out for "perfect".

Peter
-- 
Peter Williams                                   pwil3058@bigpond.net.au

"Learning, n. The kind of ignorance distinguishing the studious."
  -- Ambrose Bierce