On Wed, Nov 04, 2020 at 09:37:46AM +0100, Michal Hocko wrote: > On Tue 03-11-20 13:27:23, Roman Gushchin wrote: > > The non-hierarchical cgroup v1 mode is a legacy of early days > > of the memory controller and doesn't bring any value today. > > This is a bold statement ;) > All that we know today is that we have a warning in place to complain > loudly when somebody relies on use_hierarchy=0 with a deeper > hierarchy. For all those years we have seen _zero_ reports that would > describe a sensible usecase. > Moreover we (SUSE) have backported this warning into old distribution > kernels (since 3.0 based kernels) to extend the coverage and didn't hear > even for users who adopt new kernels only very slowly. The only report > we have seen so far was a LTP test suite which doesn't really reflect > any real life usecase.
Good to know, thank you for providing this information. I'm also not aware of any users of the non-hierarchical mode. > > Feel free to reuse the above in the changelog. > > > However, it complicates the code and creates many edge cases > > all over the memory controller code. > > > > It's a good time to deprecate it completely. > > > > Functionally this patch enabled is by default for all cgroups > > and forbids switching it off. Nothing changes if cgroup v2 is used: > > hierarchical mode was enforced from scratch. > > > > To protect the ABI memory.use_hierarchy interface is preserved > > with a limited functionality: reading always returns "1", writing > > of "1" passes silently, writing of any other value fails with > > -EINVAL and a warning to dmesg (on the first occasion). > > Yes, that makes sense. > > > Signed-off-by: Roman Gushchin <g...@fb.com> > > I do not see any problems with the patch or any left overs behind > (except for the documentation which you handle in the follow up > patches). > > Acked-by: Michal Hocko <mho...@suse.com> > > Thanks and let's see whether some last minute usecase show up. Thank you! Roman