linux-kernel - Re: [PATCH RFC 00/14] The new slab memory controller

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Date:   Wed, 2 Oct 2019 22:00:07 +0900
From:   Suleiman Souhlal <suleiman@...gle.com>
To:     Roman Gushchin <guro@...com>
Cc:     Michal Koutný <mkoutny@...e.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Michal Hocko <mhocko@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Waiman Long <longman@...hat.com>
Subject: Re: [PATCH RFC 00/14] The new slab memory controller

On Wed, Oct 2, 2019 at 11:09 AM Roman Gushchin <guro@...com> 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@...com> 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