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

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Date:   Mon, 4 Jun 2018 15:39:53 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
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

On Mon, Jun 04, 2018 at 05:59:40AM -0700, Srikar Dronamraju wrote:
> * 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.

Then your Changelog had better explain all that, no?