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Date:   Fri, 29 Oct 2021 09:37:51 +0100
From:   Mel Gorman <mgorman@suse.de>
To:     Gang Li <ligang.bdlg@bytedance.com>
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
Subject: Re: Re: [PATCH v1] sched/numa: add per-process numa_balancing
Message-ID: <20211029083751.GR3891@suse.de>
References: <20211027132633.86653-1-ligang.bdlg@bytedance.com>
 <20211028153028.GP3891@suse.de>
 <b884ad7d-48d3-fcc8-d199-9e7643552a9a@bytedance.com>
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On Fri, Oct 29, 2021 at 02:12:28PM +0800, Gang Li wrote:
> On 10/28/21 11:30 PM, Mel Gorman 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.

I'm aware that this is the behaviour of the patch as-is.

> So I
> think prctl(PR_NUMA_BALANCING, PR_SET_NUMA_BALANCING,0/1) should return
> error instead of modify 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?
> 

My point is that as it stands,
prctl(PR_NUMA_BALANCING,PR_SET_NUMA_BALANCING,1) either does nothing or
fails. If per-process numa balancing is to be introduced, it should have
meaning with the global tuning affecting default behaviour and the prctl
affecting specific behaviour.

-- 
Mel Gorman
SUSE Labs