Re: [PATCH v1 2/4] mm: memcg: merge multiple page_counters into a single structure

["T.J. Mercier" <tjmercier@google.com>]

On Fri, May 3, 2024 at 2:18 PM Roman Gushchin <roman.gushchin@linux.dev> wrote:
>
> On Fri, May 03, 2024 at 02:11:17PM -0700, Shakeel Butt wrote:
> > On Fri, May 03, 2024 at 01:18:33PM -0700, Roman Gushchin wrote:
> > [...]
> > >  enum mem_counter_type {
> > > +#ifdef CONFIG_MEMCG
> > >     MCT_MEMORY,             /* cgroup v1 and v2 */
> > >     MCT_SWAP,               /* cgroup v2 only */
> > >     MCT_MEMSW = MCT_SWAP,   /* cgroup v1 only */
> > >     MCT_KMEM,               /* cgroup v1 only */
> > >     MCT_TCPMEM,             /* cgroup v1 only */
> > > +#endif
> > > +#ifdef CONFIG_CGROUP_HUGETLB
> > > +   MCT_HUGETLB_MAX = __MCT_HUGETLB_MAX,
> > > +#endif
> > > +   __MCT_NR_ITEMS,
> > >  };
> > >
> >
> > Thanks for the awesome work. I haven't gone through all the patches yet
> > but wanted to ask a quick question. In the above enum are you trying to
> > do a union between memcg and hugetlb? It gave me a big pause to
> > understand what you are trying to do.
>
> Yep, sort of. So the page_counter structure supports N independent
> counters, where N is sufficient enough for both memcg and hugetlb cases.
>
> MCT_MEMORY, MCT_SWAP etc are used directly in the memcontrol.c code,
> while hugetlb code just indexes. MCT_HUGETLB_MAX magic is needed to define
> N at the compile time.

Where N is __MCT_NR_ITEMS for all the counter array lengths? That
doesn't look like it works if MCT_HUGETLB_MAX is small... i.e. there
is both CONFIG_MEMCG and CONFIG_CGROUP_HUGETLB and (__MCT_HUGETLB_MAX
= 1 or 3) since MCT_HUGETLB_MAX would be < MCT_TCPMEM and then
__MCT_NR_ITEMS would be wrong?

If so, what about:

enum mem_counter_type {
#ifdef CONFIG_MEMCG
    MCT_MEMORY, /* cgroup v1 and v2 */
    MCT_SWAP, /* cgroup v2 only */
    MCT_MEMSW = MCT_SWAP, /* cgroup v1 only */
    MCT_KMEM, /* cgroup v1 only */
    MCT_TCPMEM, /* cgroup v1 only */
#endif
    MCT_MEMCG_NR_ITEMS,
#ifdef CONFIG_CGROUP_HUGETLB
    MCT_HUGETLB_MAX = MCT_MEMCG_NR_ITEMS + __MCT_HUGETLB_MAX,
#else
    MCT_HUGETLB_MAX = 0,
#endif
    __MCT_NR_ITEMS = MAX(MCT_MEMCG_NR_ITEMS, MCT_HUGETLB_MAX)
};