linux-kernel - Re: [PATCH 3/3] sched,numa: do not set preferred_node on migration to a second choice node

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Date:	Mon, 14 Apr 2014 14:56:35 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	riel@...hat.com
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org, chegu_vinod@...com,
	mgorman@...e.de
Subject: Re: [PATCH 3/3] sched,numa: do not set preferred_node on migration
 to a second choice node

On Fri, Apr 11, 2014 at 01:00:29PM -0400, riel@...hat.com wrote:
> From: Rik van Riel <riel@...hat.com>
> 
> Setting the numa_preferred_node for a task in task_numa_migrate
> does nothing on a 2-node system. Either we migrate to the node
> that already was our preferred node, or we stay where we were.
> 
> On a 4-node system, it can slightly decrease overhead, by not
> calling the NUMA code as much. Since every node tends to be
> directly connected to every other node, running on the wrong
> node for a while does not do much damage.
> 
> However, on an 8 node system, there are far more bad nodes
> than there are good ones, and pretending that a second choice
> is actually the preferred node can greatly delay, or even
> prevent, a workload from converging.
> 
> The only time we can safely pretend that a second choice
> node is the preferred node is when the task is part of a
> workload that spans multiple NUMA nodes.
> 
> Signed-off-by: Rik van Riel <riel@...hat.com>
> Tested-by: Vinod Chegu <chegu_vinod@...com>
> ---
>  kernel/sched/fair.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index babd316..302facf 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -1301,7 +1301,16 @@ static int task_numa_migrate(struct task_struct *p)
>  	if (env.best_cpu == -1)
>  		return -EAGAIN;
>  
> -	sched_setnuma(p, env.dst_nid);
> +	/*
> +	 * If the task is part of a workload that spans multiple NUMA nodes,
> +	 * and is migrating into one of the workload's active nodes, remember

I read 'into' as:
  !node_isset(env.src_nid, ...) && node_isset(env.dst_nid, ...)

The code doesn't seem to do this.

> +	 * this node as the task's preferred numa node, so the workload can
> +	 * settle down.
> +	 * A task that migrated to a second choice node will be better off
> +	 * trying for a better one later. Do not set the preferred node here.
> +	 */
> +	if (p->numa_group && node_isset(env.dst_nid, p->numa_group->active_nodes))
> +		sched_setnuma(p, env.dst_nid);

OK, so I was totally confused on this one.

What I missed was that we set the primary choice over in
task_numa_placement().

I'm not really happy with the changelog; but I'm also struggling to
identify what exactly is missing. Or rather, the thing makes me
confused, and not feel like it actually explains it proper.

That said; I tend to more or less agree with the actual change, but..
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