Re: [RFC PATCH] memcg: expose children memory usage for root

[Shakeel Butt <shakeel.butt@linux.dev>]

Hi T.J.

On Thu, Jul 25, 2024 at 04:12:12PM GMT, T.J. Mercier wrote:
> On Mon, Jul 22, 2024 at 3:53 PM Shakeel Butt <shakeel.butt@linux.dev> wrote:
> >
> > Linux kernel does not expose memory.current on the root memcg and there
> > are applications which have to traverse all the top level memcgs to
> > calculate the total memory charged in the system. This is more expensive
> > (directory traversal and multiple open and reads) and is racy on a busy
> > machine. As the kernel already have the needed information i.e. root's
> > memory.current, why not expose that?
> >
> > However root's memory.current will have a different semantics than the
> > non-root's memory.current as the kernel skips the charging for root, so
> > maybe it is better to have a different named interface for the root.
> > Something like memory.children_usage only for root memcg.
> >
> > Now there is still a question that why the kernel does not expose
> > memory.current for the root. The historical reason was that the memcg
> > charging was expensice and to provide the users to bypass the memcg
> > charging by letting them run in the root. However do we still want to
> > have this exception today? What is stopping us to start charging the
> > root memcg as well. Of course the root will not have limits but the
> > allocations will go through memcg charging and then the memory.current
> > of root and non-root will have the same semantics.
> >
> > This is an RFC to start a discussion on memcg charging for root.
> 
> Hi Shakeel,
> 
> Since the root already has a page_counter I'm not opposed to this new
> file as it doesn't increase the page_counter depth for children.
> However I don't currently have any use-cases for it that wouldn't be
> met by memory.stat in the root.

I think difference would be getting a single number versus accumulating
different fields in memory.stat to get that number (memory used by
root's children) which might be a bit error prone.

> 
> As far as charging, I've only ever seen kthreads and init in the root.
> You have workloads that run there?

No workloads in root. The charging is only to make the semanctics of
root's memory.current same as its descendants.

Thanks,
Shakeel