linux-kernel - Re: [PATCH] sched/fair: use dst group while checking imbalance for NUMA balancer

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Message-ID: <20200907092717.GD3117@suse.de>
Date:   Mon, 7 Sep 2020 10:27:17 +0100
From:   Mel Gorman <mgorman@...e.de>
To:     Barry Song <song.bao.hua@...ilicon.com>
Cc:     mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        bsegall@...gle.com, linux-kernel@...r.kernel.org,
        linuxarm@...wei.com, Mel Gorman <mgorman@...hsingularity.net>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Valentin Schneider <valentin.schneider@....com>,
        Phil Auld <pauld@...hat.com>, Hillf Danton <hdanton@...a.com>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH] sched/fair: use dst group while checking imbalance for
 NUMA balancer

On Mon, Sep 07, 2020 at 07:27:08PM +1200, Barry Song wrote:
> Something is wrong. In find_busiest_group(), we are checking if src has
> higher load, however, in task_numa_find_cpu(), we are checking if dst
> will have higher load after balancing. It seems it is not sensible to
> check src.
> It maybe cause wrong imbalance value, for example, if
> dst_running = env->dst_stats.nr_running + 1 results in 3 or above, and
> src_running = env->src_stats.nr_running - 1 results in 1;
> The current code is thinking imbalance as 0 since src_running is smaller
> than 2.
> This is inconsistent with load balancer.
> 

It checks the conditions if the move was to happen. Have you evaluated
this for a NUMA balancing load and confirmed it a) balances properly and
b) does not increase the scan rate trying to "fix" the problem?

-- 
Mel Gorman
SUSE Labs