On Thu, Oct 29, 2020 at 04:39:21PM +0100, Michal Koutny wrote:

Hi.

On Mon, Oct 26, 2020 at 04:13:26PM -0700, Roman Gushchin guro@fb.com wrote:

...

Please note, that in the non-hierarchical mode all objcgs are always reparented to the root memory cgroup, even if the hierarchy has more than 1 level. This patch doesn't change it.

The patch also doesn't affect how the hierarchical mode is working, which is the only sane and truly supported mode now.

I agree with the patch and you can add Reviewed-by: Michal Koutný mkoutny@suse.com

However, it effectively switches any users of root.use_hierarchy=0 (if there are any, watching the counters of root memcg) into root.use_hierarchy=1. So I'd show them the warning even with a single level of cgroups, i.e. add this hunk

It's only partially true. The main difference between the hierarchical and non-hierarchical mode on the following simple example

/ | A / \ B C

is whether A's memory limits are applied to B, and this is not gonna change. However you're right, it will change some root cgroup's numbers.

@@ -5356,12 +5356,11 @@ page_counter_init(&memcg->kmem, &root_mem_cgroup->kmem); page_counter_init(&memcg->tcpmem, &root_mem_cgroup->tcpmem); /*

•  * Deeper hierachy with use_hierarchy == false doesn't make
  

•  * Hierachy with use_hierarchy == false doesn't make
  

  • much sense so let cgroup subsystem know about this
  • unfortunate state in our controller.
  */

• if (parent != root_mem_cgroup)
  

• 	memory_cgrp_subsys.broken_hierarchy = true;
  

• memory_cgrp_subsys.broken_hierarchy = true;
  

  }

/* The following stuff does not apply to the root */

What do you think?

I think it's in a good direction of deprecating the non-hierarchical mode. Shakeel did propose it too.

I'd also change the displayed message to something similar to we print for kmem.limit_in_bytes: pr_warn_once("kmem.limit_in_bytes is deprecated and will be removed. " "Please report your usecase to linux-mm@kvack.org if you " "depend on this functionality.\n");

Thanks!