linux-kernel - Re: [RFC 2/4] sched/numa: replace runnable_load_avg by load

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Date:   Thu, 13 Feb 2020 17:11:23 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Phil Auld <pauld@...hat.com>, Hillf Danton <hdanton@...a.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Parth Shah <parth@...ux.ibm.com>,
        Valentin Schneider <valentin.schneider@....com>
Subject: Re: [RFC 2/4] sched/numa: replace runnable_load_avg by load_avg

On Thu, 13 Feb 2020 at 16:14, Mel Gorman <mgorman@...hsingularity.net> wrote:
>
> On Thu, Feb 13, 2020 at 10:00:26AM -0500, Phil Auld wrote:
> > On Thu, Feb 13, 2020 at 01:46:55PM +0000 Mel Gorman wrote:
> > > On Thu, Feb 13, 2020 at 09:16:58PM +0800, Hillf Danton wrote:
> > > > > -       load = task_h_load(env->p);
> > > > > -       dst_load = env->dst_stats.load + load;
> > > > > -       src_load = env->src_stats.load - load;
> > > > > -
> > > > >         /*
> > > > > -        * If the improvement from just moving env->p direction is better
> > > > > -        * than swapping tasks around, check if a move is possible.
> > > > > +        * If dst node has spare capacity, then check if there is an
> > > > > +        * imbalance that would be overruled by the load balancer.
> > > > >          */
> > > > > -       maymove = !load_too_imbalanced(src_load, dst_load, env);
> > > > > +       if (env->dst_stats.node_type == node_has_spare) {
> > > > > +               unsigned int imbalance;
> > > > > +               int src_running, dst_running;
> > > > > +
> > > > > +               /* Would movement cause an imbalance? */
> > > > > +               src_running = env->src_stats.nr_running - 1;
> > > > > +               dst_running = env->src_stats.nr_running + 1;
> > > > > +               imbalance = max(0, dst_running - src_running);
> > > >
> > > > Have trouble working out why 2 is magician again to make your test data nicer :P
> > > >
> > >
> > > This is calculating what the nr_running would be after the move and
> > > checking if an imbalance exists after the move forcing the load balancer
> > > to intervene.
> >
> > Isn't that always going to work out to 2?
> >
>
> Crap, stupid cut and paste moving between source trees. Yes, this is
> broken.

On the load balance side we have 2 rules when NUMA groups has spare capacity:
- ensure that the diff between src and dst nr_running < 2
- if src_nr_running is lower than 2, allow a degree of imbalance of 2
instead of 1

Your test doesn't explicitly ensure that the 1 condition is met

That being said, I'm not sure it's really a wrong thing ? I mean
load_balance will probably try to pull back some tasks on src but as
long as it is not a task with dst node as preferred node, it should
not be that harmfull

>
> --
> Mel Gorman
> SUSE Labs