On Wed, Oct 2, 2019 at 11:09 AM Roman Gushchin <guro@xxxxxx> wrote: > > On Tue, Oct 01, 2019 at 05:12:02PM +0200, Michal Koutný wrote: > > On Thu, Sep 05, 2019 at 02:45:44PM -0700, Roman Gushchin <guro@xxxxxx> wrote: > > > Roman Gushchin (14): > > > [...] > > > mm: memcg/slab: use one set of kmem_caches for all memory cgroups > > From that commit's message: > > > > > 6) obsoletes kmem.slabinfo cgroup v1 interface file, as there are > > > no per-memcg kmem_caches anymore (empty output is printed) > > > > The empty file means no allocations took place in the particular cgroup. > > I find this quite a surprising change for consumers of these stats. > > > > I understand obtaining the same data efficiently from the proposed > > structures is difficult, however, such a change should be avoided. (In > > my understanding, obsoleted file ~ not available in v2, however, it > > should not disappear from v1.) > > Well, my assumption is that nobody is using this file for anything except > debugging purposes (I might be wrong, if somebody has an automation based > on it, please, let me know). A number of allocations of each type per memory > cgroup is definitely a useful debug information, but currently it barely works > (displayed numbers show mostly the number of allocated pages, not the number > of active objects). We can support it, but it comes with the price, and > most users don't really need it. So I don't think it worth it to make all > allocations slower just to keep some debug interface working for some > cgroup v1 users. Do you have examples when it's really useful and worth > extra cpu cost? > > Unfortunately, we can't enable it conditionally, as a user can switch > between cgroup v1 and cgroup v2 memory controllers dynamically. kmem.slabinfo has been absolutely invaluable for debugging, in my experience. I am however not aware of any automation based on it. Maybe it might be worth adding it to cgroup v2 and have a CONFIG option to enable it? -- Suleiman
