Re: [RFC PATCH v3 0/3] sched: simplify the select_task_rq_fair()

From: Michael Wang
Date: Wed Feb 20 2013 - 23:51:47 EST

On 02/20/2013 06:49 PM, Ingo Molnar wrote:
> The changes look clean and reasoable, any ideas exactly *why* it
> speeds up?
> I.e. are there one or two key changes in the before/after logic
> and scheduling patterns that you can identify as causing the
> speedup?

Hi, Ingo

Thanks for your reply, please let me point out the key changes here
(forgive me for haven't wrote a good description in cover).

The performance improvement from this patch set is:
1. delay the invoke on wake_affine().
2. save the circle to gain proper sd.

The second point is obviously, and will benefit a lot when the sd
topology is deep (NUMA is suppose to make it deeper on large system).

So in my testing on a 12 cpu box, actually most of the benefit comes
from the first point, and please let me introduce it in detail.

The old logical when locate affine_sd is:

if prev_cpu != curr_cpu
if wake_affine()
prev_cpu = curr_cpu
new_cpu = select_idle_sibling(prev_cpu)
return new_cpu

The new logical is same to the old one if prev_cpu == curr_cpu, so let's
simplify the old logical like:

if wake_affine()
new_cpu = select_idle_sibling(curr_cpu)
new_cpu = select_idle_sibling(prev_cpu)

return new_cpu

Actually that doesn't make sense.

I think wake_affine() is trying to check whether move a task from
prev_cpu to curr_cpu will break the balance in affine_sd or not, but why
won't break balance means curr_cpu is better than prev_cpu for searching
the idle cpu?

So the new logical in this patch set is:

new_cpu = select_idle_sibling(prev_cpu)
if idle_cpu(new_cpu)
return new_cpu

new_cpu = select_idle_sibling(curr_cpu)
if idle_cpu(new_cpu) {
if wake_affine()
return new_cpu

return prev_cpu

And now, unless we are really going to move load from prev_cpu to
curr_cpu, we won't use wake_affine() any more.

So we avoid wake_affine() when system load is low or high, for middle
load, the worst cases is when failed to locate idle cpu in prev_cpu
topology but succeed to locate one in curr_cpu's, but that's rarely
happen and the benchmark results proved that point.

Some comparison below:

1. system load is low
old logical cost:
new logical cost:

2. system load is high
old logical cost:
new logical cost:

3. system load is middle
don't know

1 save the cost of wake_affine(), 3 could be proved by benchmark that no
regression at least.

For 2, it's the comparison between wake_affine() and
select_idle_sibling(), since the system load is high, wake_affine() cost
far more than select_idle_sibling(), and we saved many according to the
benchmark results.

> Such changes also typically have a chance to cause regressions
> in other workloads - when that happens we need this kind of
> information to be able to enact plan-B.

The benefit comes from avoiding unnecessary works, and the patch set is
suppose to only reduce the cost of key function with least logical
changing, I could not promise it benefit all the workloads, but till
now, I've not found regression.

Michael Wang

> Thanks,
> Ingo

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