Re: [PATCH 0/4] sched: remove cpu_load decay

[Morten Rasmussen <morten.rasmussen@arm.com>]

On Sat, Dec 14, 2013 at 01:27:59PM +0000, Alex Shi wrote:
> On 12/14/2013 04:03 AM, Peter Zijlstra wrote:
> > 
> > 
> > I had a quick peek at the actual patches.
> > 
> > afaict we're now using weighted_cpuload() aka runnable_load_avg as the
> > ->cpu_load. Whatever happened to also using the blocked_avg?

AFAICT, ->cpu_load is actually a snapshot value of weigthed_cpuload()
that gets updated occasionally. That has been the case since b92486cb.
By removing the cpu_load indexes {source,target}_load are now comparing
an old snapshot of weighted_cpuload() with the current value. I don't
think that really makes sense. weighted_cpuload() may change rapidly
when tasks are enqueued or dequeued so the old snapshot doesn't have
much meaning in my opinion. Maybe I'm missing something?

Comparing cpu_load indexes with different decay rates in {source,
target}_load() sort of make sense as it makes load-balancing decisions
more conservative.

If we can indeed remove decayed cpu_load there is more code that should
be revisited and potentially be ripped out. {source,target}_load() could
probably be reduced to weighted_cpuload(), which would change the
load-balance behaviour. However, these patches already affect
load-balancing as indicated by the fix in patch 4.

I believe we have discussed using blocked_load_avg in weighted_cpuload()
in the past. While it seems to be the right thing to include it, it
causes problems related to the priority scaling of the task loads.
If you include a blocked load in the weighted_cpuload() and have tiny
(very low cpu utilization) task running at very high priority, your
weighted_cpuload() will be quite high and force other normal priority
tasks away from the cpu and leaving the cpu idle most of the time.

> 
> When enabling the sched_avg in load balance, I didn't find any positive
> testing result for several blocked_avg trying, just few regression. :(
> 
> And since this patchset is almost clean up only, no blocked_load_avg
> trying again...

My worry here is that I don't really understand why the current code
works when the decayed cpu_load has been removed.

> > 
> > I totally hate patch 4; it seems like a random hack to make up for the
> > lack of blocked_avg.
> 
> Yes, this bias criteria seems a bit arbitrary. :)

This is why I think {source, target}_load() and their use need to be
reconsidered.

Morten