Re: Re: [PATCH v4] mm: oom: introduce cpuset oom

[Gang Li]

On 2023/4/11 20:23, Michal Koutný wrote:
> Hello.
>
> On Tue, Apr 11, 2023 at 02:58:15PM +0800, Gang Li <ligang.bdlg@xxxxxxxxxxxxx> wrote:
> > +	cpuset_for_each_descendant_pre(cs, pos_css, &top_cpuset) {
> > +		if (nodes_equal(cs->mems_allowed, task_cs(current)->mems_allowed)) {
> > +			css_task_iter_start(&(cs->css), CSS_TASK_ITER_PROCS, &it);
> > +			while (!ret && (task = css_task_iter_next(&it)))
> > +				ret = fn(task, arg);
> > +			css_task_iter_end(&it);
> > +		}
> > +	}
> > +	rcu_read_unlock();
> > +	cpuset_read_unlock();
> > +	return ret;
> > +}
>
> I see this traverses all cpusets without the hierarchy actually
> mattering that much. Wouldn't the CONSTRAINT_CPUSET better achieved by
> globally (or per-memcg) scanning all processes and filtering with:

Oh I see, you mean scanning all processes in all cpusets and scanning
all processes globally are equivalent.

> 	nodes_intersect(current->mems_allowed, p->mems_allowed

Perhaps it would be better to use nodes_equal first, and if no suitable
victim is found, then downgrade to nodes_intersect?

NUMA balancing mechanism tends to keep memory on the same NUMA node, and
if the selected victim's memory happens to be on a node that does not
intersect with the current process's node, we still won't be able to
free up any memory.

In this example:

A->mems_allowed: 0,1
B->mems_allowed: 1,2
nodes_intersect(A->mems_allowed, B->mems_allowed) == true

Memory Distribution:
+=======+=======+=======+
| Node0 | Node1 | Node2 |
+=======+=======+=======+
| A     |       |       |
+-------+-------+-------+
|       |       |B      |
+-------+-------+-------+

Process A invoke oom, then kill B.
But A still can't get any free mem on Node0 and 1.

> (`current` triggers the OOM, `p` is the iterated task)
> ?
>
> Thanks,
> Michal