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

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Date:   Thu, 13 Feb 2020 17:02:21 +0000
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Vincent Guittot <vincent.guittot@...aro.org>
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, Feb 13, 2020 at 05:38:31PM +0100, Vincent Guittot wrote:
> > > 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
> >
> > My thinking was that if source has as many or more running tasks than
> > the destination *after* the move that it's not harmful and does not add
> > work for the load balancer.
> 
> load_balancer will see an imbalance but fbq_classify_group/queue
> should be there to prevent from pulling back tasks that are on the
> preferred node but only other tasks
> 

Yes, exactly. Between fbq_classify and migrate_degrades_locality, I'm
expecting that the load balancer will only override NUMA balancing when
there is no better option. When the imbalance check, I want to avoid
the situation where NUMA balancing moves a task for locality, LB pulls
it back for balance, NUMA retries the move etc because it's stupid. The
locality matters but being continually dequeue/enqueue is unhelpful.

While there might be grounds for relaxing the degree an imbalance is
allowed across SD domains, I am avoiding looking in that direction again
until the load balancer and NUMA balancer stop overriding each other for
silly reasons (or the NUMA balancer fighting itself which can happen).

-- 
Mel Gorman
SUSE Labs