linux-kernel - Re: [PATCH 04/19] sched/numa: Set preferred_node based on best

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Message-Id: <20180604125939.GB38574@linux.vnet.ibm.com>
Date:   Mon, 4 Jun 2018 05:59:40 -0700
From:   Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Rik van Riel <riel@...riel.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 04/19] sched/numa: Set preferred_node based on best_cpu

* Peter Zijlstra <peterz@...radead.org> [2018-06-04 14:23:36]:

> OK, the above matches the description, but I'm puzzled by the remainder:
>
> >
> > -		if (ng->active_nodes > 1 && numa_is_active_node(env.dst_nid, ng))
> > -			sched_setnuma(p, env.dst_nid);
> > +		if (nid != p->numa_preferred_nid)
> > +			sched_setnuma(p, nid);
> >  	}
>
> That seems to entirely loose the active_node thing, or are you saying
> best_cpu already includes that? (Changelog could use a little help there
> I suppose)

I think checking for active_nodes before calling sched_setnuma was a
mistake.

Before this change, we may be retaining numa_preferred_nid to be the
source node while we select another node with better numa affinity to
run on. So we are creating a situation where we force a thread to run on
a node which is not going to be its preferred_node. So in the course of
regular load balancing, this task might then be moved to set
preferred_node which is actually not the preferred_node.