Re: Re: [PATCH v1] sched/numa: add per-process numa_balancing

[Gang Li]

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. 
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?