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

[Shakeel Butt]

On Mon, Jun 22, 2020 at 11:02 AM Roman Gushchin <guro@xxxxxx> 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@xxxxxx> 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@xxxxxx>
> > > Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> > > Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx>
> >
> > 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.

> 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?