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