linux-kernel - Re: [PATCH 4/4] sched,fair: remove effective

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Date:   Mon, 26 Jun 2017 15:34:49 -0400
From:   Rik van Riel <riel@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, jhladky@...hat.com, mingo@...nel.org,
        mgorman@...e.de
Subject: Re: [PATCH 4/4] sched,fair: remove effective_load

On Mon, 2017-06-26 at 18:12 +0200, Peter Zijlstra wrote:
> On Mon, Jun 26, 2017 at 11:20:54AM -0400, Rik van Riel wrote:
> 
> > Oh, indeed.  I guess in wake_affine() we should test
> > whether the CPUs are in the same NUMA node, rather than
> > doing cpus_share_cache() ?
> 
> Well, since select_idle_sibling() is on LLC; the early test on
> cpus_share_cache(prev,this) seems to actually make sense.
> 
> But then cutting out all the other bits seems wrong. Not in the least
> because !NUMA_BALACING should also still keep working.

Even when !NUMA_BALANCING, I suspect it makes little sense
to compare the loads just one the cores in question, since
select_idle_sibling() will likely move the task somewhere
else.

I suspect we want to compare the load on the whole LLC
for that reason, even with NUMA_BALANCING disabled.

> > Or, alternatively, have an update_numa_stats() variant
> > for numa_wake_affine() that works on the LLC level?
> 
> I think we want to retain the existing behaviour for everything
> larger than LLC, and when NUMA_BALANCING, smaller than NUMA.

What do you mean by this, exactly?

How does the "existing behaviour" of only looking at
the load on two cores make sense when doing LLC-level
task placement?

> Also note that your use of task_h_load() in the new numa thing
> suffers
> from exactly the problem effective_load() is trying to solve.

Are you saying task_h_load is wrong in task_numa_compare()
too, then?  Should both use effective_load()?