linux-kernel - Re: [PATCH 0/2] Support hugetlb charge moving at task migration

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Date:   Fri, 8 Oct 2021 13:55:23 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc:     akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/2] Support hugetlb charge moving at task migration

On Fri 08-10-21 17:17:12, Baolin Wang wrote:
> 
> 
> On 2021/10/8 15:12, Michal Hocko wrote:
> > On Thu 07-10-21 23:39:15, Baolin Wang wrote:
> > > Hi Michal,
> > > 
> > > (Sorry for late reply due to my holidays)
> > > On 2021/9/30 18:46, Michal Hocko wrote:
> > > > On Wed 29-09-21 18:19:26, Baolin Wang wrote:
> > > > > Hi,
> > > > > 
> > > > > Now in the hugetlb cgroup, charges associated with a task aren't moved
> > > > > to the new hugetlb cgroup at task migration, which is odd for hugetlb
> > > > > cgroup usage.
> > > > 
> > > > Could you elaborate some more about the usecase and/or problems you see
> > > > with the existing semantic?
> > > 
> > > The problems is that, it did not check if the tasks can move to the new
> > > hugetlb cgroup if the new hugetlb cgroup has a limitation, and the hugetlb
> > > cgroup usage is incorrect when moving tasks among hugetlb cgroups.
> > 
> > Could you be more specific please? What do you mean by cgroup usage is
> > incorrect? Ideally could you describe your usecase?
> 
> Sorry for confusing, what I mean is, when tasks from one hugetlb cgroup are
> migrated to a new hugetlb cgroup, the new hugetlb cgroup's hugetlb page
> usage is not increased accordingly.

Which is a perferctly reasonable behavior as the memory has been
consumed from the original cgroup and it will be freed there as well.
Migrating to a new cgroup doesn't imply all the resources to be migrated
as well.

> The issue I found is just from my
> testing for the hugetlb cgroup, and I think this is not resonable if we've
> already set a hugetlb limitation for a cgroup, but we always ignore it when
> tasks migration among hugetlb cgroups.

I would like to learn more about why you consider this unreasonable.
This will likely depend on the reason why you want/need to migrate task.
If you want to move a task to completely new resource domain (read a
completely different cgroup subtree) then I can imagine you want to
leave all the resources but this will have problems with other resource
controllers - e.g. memory cgroup v2 which doesn't allow charge migration
either.
-- 
Michal Hocko
SUSE Labs