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Date:   Fri, 7 Sep 2018 13:33:09 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
        Ingo Molnar <mingo@...nel.org>,
        Rik van Riel <riel@...riel.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/4] sched/numa: Do not move imbalanced load purely on
 the basis of an idle CPU

On Fri, Sep 07, 2018 at 11:11:39AM +0100, Mel Gorman wrote:
> Commit 305c1fac3225 ("sched/numa: Evaluate move once per node")
> restructured how task_numa_compare evaluates load but there is an anomaly.
> task_numa_find_cpu() checks if the load balance between too nodes is too
> imbalanced with the intent of only swapping tasks if it would improve
> the balance overall. However, if an idle CPU is encountered, the task is
> still moved if it's the best improvement but an idle cpu is always going
> to appear to be the best improvement.
> 
> If a machine is lightly loaded such that all tasks can fit on one node then
> the idle CPUs are found and the tasks migrate to one socket.  From a NUMA
> perspective, this seems intuitively great because memory accesses are all
> local but there are two counter-intuitive effects.
> 
> First, the load balancer may move tasks so the machine is more evenly
> utilised and conflict with automatic NUMA balancing which may respond by
> scanning more frequently and increasing overhead.  Second, sockets usually
> have their own memory channels so using one socket means that fewer
> channels are available yielding less memory bandwidth overall. For
> memory-bound tasks, it can be beneficial to migrate to another socket and
> migrate the data to increase bandwidth even though the accesses are remote
> in the short term.

> ---
>  kernel/sched/fair.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index d59d3e00a480..d4c289c11012 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -1560,7 +1560,7 @@ static bool task_numa_compare(struct task_numa_env *env,
>  		goto unlock;
>  
>  	if (!cur) {
> -		if (maymove || imp > env->best_imp)
> +		if (maymove)
>  			goto assign;
>  		else
>  			goto unlock;

Srikar's patch here:

  http://lkml.kernel.org/r/1533276841-16341-4-git-send-email-srikar@linux.vnet.ibm.com

Also frobs this condition, but in a less radical way. Does that yield
similar results?

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