Messages in this thread | | | Date | Tue, 17 Dec 2013 14:04:57 +0000 | From | Morten Rasmussen <> | Subject | Re: [PATCH 0/4] sched: remove cpu_load decay |
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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
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