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From:   =?UTF-8?B?5p2O5riv?= <ligang.bdlg@bytedance.com>
Date:   Fri, 29 Oct 2021 15:48:50 +0800
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Subject: Re: [External] Re: [PATCH v1] sched/numa: add per-process numa_balancing
To:     Mel Gorman <mgorman@suse.de>
Cc:     Jonathan Corbet <corbet@lwn.net>, Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        linux-api@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, linux-doc@vger.kernel.org
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On Thu, Oct 28, 2021 at 11:30 PM Mel Gorman <mgorman@suse.de> wrote:
>
> That aside though, the configuration space could be better. It's possible
> to selectively disable NUMA balance but not selectively enable because
> prctl is disabled if global NUMA balancing is disabled. That could be
> somewhat achieved by having a default value for mm->numa_balancing based on
> whether the global numa balancing is disabled via command line or sysctl
> and enabling the static branch if prctl is used with an informational
> message. This is not the only potential solution but as it stands,
> there are odd semantic corner cases. For example, explicit enabling
> of NUMA balancing by prctl gets silently revoked if numa balancing is
> disabled via sysctl and prctl(PR_NUMA_BALANCING, PR_SET_NUMA_BALANCING,
> 1) means nothing.

static void task_tick_fair(struct rq *rq, struct task_struct *curr, int queued)
{
    ...
    if (static_branch_unlikely(&sched_numa_balancing))
        task_tick_numa(rq, curr);
    ...
}

static void task_tick_numa(struct rq *rq, struct task_struct *curr)
{
    ...
    if (!READ_ONCE(curr->mm->numa_balancing))
        return;
    ...
}

When global numa_balancing is disabled, mm->numa_balancing is useless.
So I think
prctl(PR_NUMA_BALANCING, PR_SET_NUMA_BALANCING,0/1) should return an
error instead of modifying mm->numa_balancing.

Is it reasonable that prctl(PR_NUMA_BALANCING,PR_SET_NUMA_BALANCING,0/1)
can still change the value of mm->numa_balancing when global numa_balancing is
disabled?