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

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

On Thu, Jul 25, 2024 at 04:20:45PM GMT, Yosry Ahmed 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.
> 
> I vaguely remember when running some netperf tests (tcp_rr?) in a
> cgroup that the performance decreases considerably with every level
> down the hierarchy. I am assuming that charging was a part of the
> reason. If that's the case, charging the root will be similar to
> moving all workloads one level down the hierarchy in terms of charging
> overhead.

No, the workloads running in non-root memcgs will not see any
difference. Only the workloads running in root will see charging
overhead.