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SubjectRe: [RESEND PATCH V5 0/8] remove cpu_load idx
On Wed, Apr 16, 2014 at 03:43:21AM +0100, Alex Shi wrote:
> In the cpu_load decay usage, we mixed the long term, short term load with
> balance bias, randomly pick a big or small value according to balance
> destination or source.

I disagree that it is random. min()/max() in {source,target}_load()
provides a conservative bias to the load estimate that should prevent us
from trying to pull tasks from the source cpu if its current load is
just a temporary spike. Likewise, we don't try to pull tasks to the
target cpu if the load is just a temporary drop.

> This mix is wrong, the balance bias should be based
> on task moving cost between cpu groups, not on random history or instant load.

Your patch set actually changes everything to be based on the instant
load alone. rq->cfs.runnable_load_avg is updated instantaneously when
tasks are enqueued and deqeueue, so this load expression is quite volatile.

What do you mean by "task moving cost"?

> History load maybe diverage a lot from real load, that lead to incorrect bias.
>
> like on busy_idx,
> We mix history load decay and bias together. The ridiculous thing is, when
> all cpu load are continuous stable, long/short term load is same. then we
> lose the bias meaning, so any minimum imbalance may cause unnecessary task
> moving. To prevent this funny thing happen, we have to reuse the
> imbalance_pct again in find_busiest_group(). But that clearly causes over
> bias in normal time. If there are some burst load in system, it is more worse.

Isn't imbalance_pct only used once in the periodic load-balance path?

It is not clear to me what the over bias problem is. If you have a
stable situation, I would expect the long and short term load to be the
same?

> As to idle_idx:
> Though I have some cencern of usage corretion,
> https://lkml.org/lkml/2014/3/12/247 but since we are working on cpu
> idle migration into scheduler. The problem will be reconsidered. We don't
> need to care it too much now.
>
> In fact, the cpu_load decays can be replaced by the sched_avg decay, that
> also decays load on time. The balance bias part can fullly use fixed bias --
> imbalance_pct, which is already used in newly idle, wake, forkexec balancing
> and numa balancing scenarios.

As I have said previously, I agree that cpu_load[] is somewhat broken in
its current form, but I don't see how removing it and replacing it with
the instantaneous cpu load solves the problems you point out.

The current cpu_load[] averages the cpu_load over time, while
rq->cfs.runnable_load_avg is the sum of the currently runnable tasks'
load_avg_contrib. The former provides a long term view of the cpu_load,
the latter does not. It can change radically in an instant. I'm
therefore a bit concerned about the stability of the load-balance
decisions. However, since most decisions are based on cpu_load[0]
anyway, we could try setting LB_BIAS to false as Peter suggests and see
what happens.

Morten


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