On Fri, Jul 26, 2024 at 8:47 AM Shakeel Butt <shakeel.butt@xxxxxxxxx> wrote: > > 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@xxxxxxxxx> 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. Yeah that makes sense, I get how it'd be nicer to do just one read in the root instead of digging into all the children. I just meant to say that when looking at the root, currently I only care about a particular stat (e.g. file_mapped) instead of the whole usage. > > 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
