Re: [PATCH v6 10/19] mm: memcg/slab: deprecate memory.kmem.slabinfo

[Shakeel Butt <shakeelb@google.com>]

On Mon, Jun 22, 2020 at 11:25 AM Roman Gushchin <guro@fb.com> wrote:
>
> On Mon, Jun 22, 2020 at 11:09:47AM -0700, Shakeel Butt wrote:
> > On Mon, Jun 22, 2020 at 11:02 AM Roman Gushchin <guro@fb.com> wrote:
> > >
> > > On Mon, Jun 22, 2020 at 10:12:46AM -0700, Shakeel Butt wrote:
> > > > On Mon, Jun 8, 2020 at 4:07 PM Roman Gushchin <guro@fb.com> wrote:
> > > > >
> > > > > Deprecate memory.kmem.slabinfo.
> > > > >
> > > > > An empty file will be presented if corresponding config options are
> > > > > enabled.
> > > > >
> > > > > The interface is implementation dependent, isn't present in cgroup v2,
> > > > > and is generally useful only for core mm debugging purposes. In other
> > > > > words, it doesn't provide any value for the absolute majority of users.
> > > > >
> > > > > A drgn-based replacement can be found in tools/cgroup/slabinfo.py .
> > > > > It does support cgroup v1 and v2, mimics memory.kmem.slabinfo output
> > > > > and also allows to get any additional information without a need
> > > > > to recompile the kernel.
> > > > >
> > > > > If a drgn-based solution is too slow for a task, a bpf-based tracing
> > > > > tool can be used, which can easily keep track of all slab allocations
> > > > > belonging to a memory cgroup.
> > > > >
> > > > > Signed-off-by: Roman Gushchin <guro@fb.com>
> > > > > Acked-by: Johannes Weiner <hannes@cmpxchg.org>
> > > > > Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
> > > >
> > > > Hi Roman,
> > > >
> > > > I am not against removing the memory.kmem.slabinfo interface but I
> > > > would like to have an alternative solution more accessible than
> > > > tools/cgroup/slabinfo.py.
> > > >
> > > > In our case, we don't have ssh access and if we need something for
> > > > debugging, it is much more preferable to provide a file to read to
> > > > SREs. After the review, that file will be added to a whitelist and
> > > > then we can directly read that file through automated tools without
> > > > approval for each request.
> > > >
> > > > I am just wondering if a file interface can be provided for whatever
> > > > tools/cgroup/slabinfo.py is providing.
> > > >
> > > > Shakeel
> > >
> > > Hello, Shakeel!
> > >
> > > I understand your point, but Idk how much we wanna make this code a part
> > > of the kernel and the cgroup interface.
> >
> > No need for the cgroup interface. I was thinking of a new interface
> > like /proc/slabinfo_full which tells active objects for each
> > kmem_cache and memcg pair.
>
> To me it's a perfect example where tools like drgn and bpf shine.
> They are more flexible and do not blow the kernel up with
> the debug-only code.
>
> >
> > > The problem is that reading
> > > from it will be really slow in comparison to all other cgroup interface
> > > files. Idk if Google's version of SLAB has a list of all slab pages,
> > > but if not (as in generic SLUB case), it requires scanning of the whole RAM.
> >
> > That's a bummer. Does drgn-based script scan the whole RAM?
>
> To be precise, not over all RAM, but over all struct pages.
> Unfortunately, there is no better option with SLUB, as there is no
> comprehensive list of slab pages available. So the only option is to scan
> over all pages with PageSlab flag set.
>

So, SLUB does not have any field available in the struct page to
support the list of slab pages?

Anyways, that's a separate discussion.

Reviewed-by: Shakeel Butt <shakeelb@google.com>