Re: [RFC PATCH bpf-next v2 00/11] mm, bpf: Add BPF into /proc/meminfo

[Yafang Shao <laoar.shao@gmail.com>]

On Wed, Jan 18, 2023 at 1:25 AM Alexei Starovoitov
<alexei.starovoitov@gmail.com> wrote:
>
> On Fri, Jan 13, 2023 at 3:53 AM Yafang Shao <laoar.shao@gmail.com> wrote:
> >
> > On Fri, Jan 13, 2023 at 5:05 AM Alexei Starovoitov
> > <alexei.starovoitov@gmail.com> wrote:
> > >
> > > On Thu, Jan 12, 2023 at 7:53 AM Yafang Shao <laoar.shao@gmail.com> wrote:
> > > >
> > > > Currently there's no way to get BPF memory usage, while we can only
> > > > estimate the usage by bpftool or memcg, both of which are not reliable.
> > > >
> > > > - bpftool
> > > >   `bpftool {map,prog} show` can show us the memlock of each map and
> > > >   prog, but the memlock is vary from the real memory size. The memlock
> > > >   of a bpf object is approximately
> > > >   `round_up(key_size + value_size, 8) * max_entries`,
> > > >   so 1) it can't apply to the non-preallocated bpf map which may
> > > >   increase or decrease the real memory size dynamically. 2) the element
> > > >   size of some bpf map is not `key_size + value_size`, for example the
> > > >   element size of htab is
> > > >   `sizeof(struct htab_elem) + round_up(key_size, 8) + round_up(value_size, 8)`
> > > >   That said the differece between these two values may be very great if
> > > >   the key_size and value_size is small. For example in my verifaction,
> > > >   the size of memlock and real memory of a preallocated hash map are,
> > > >
> > > >   $ grep BPF /proc/meminfo
> > > >   BPF:                 350 kB  <<< the size of preallocated memalloc pool
> > > >
> > > >   (create hash map)
> > > >
> > > >   $ bpftool map show
> > > >   41549: hash  name count_map  flags 0x0
> > > >         key 4B  value 4B  max_entries 1048576  memlock 8388608B
> > > >
> > > >   $ grep BPF /proc/meminfo
> > > >   BPF:               82284 kB
> > > >
> > > >   So the real memory size is $((82284 - 350)) which is 81934 kB
> > > >   while the memlock is only 8192 kB.
> > >
> > > hashmap with key 4b and value 4b looks artificial to me,
> > > but since you're concerned with accuracy of bpftool reporting,
> > > please fix the estimation in bpf_map_memory_footprint().
> >
> > I thought bpf_map_memory_footprint() was deprecated, so I didn't try
> > to fix it before.
>
> It's not deprecated. It's trying to be accurate.
> See bpf_map_value_size().
> In the past we had to be precise when we calculated the required memory
> before we allocated and that was causing ongoing maintenance issues.
> Now bpf_map_memory_footprint() is an estimate for show_fdinfo.
> It can be made more accurate for this map with corner case key/value sizes.
>

Thanks for the clarification.

> > > You're correct that:
> > >
> > > > size of some bpf map is not `key_size + value_size`, for example the
> > > >   element size of htab is
> > > >   `sizeof(struct htab_elem) + round_up(key_size, 8) + round_up(value_size, 8)`
> > >
> > > So just teach bpf_map_memory_footprint() to do this more accurately.
> > > Add bucket size to it as well.
> > > Make it even more accurate with prealloc vs not.
> > > Much simpler change than adding run-time overhead to every alloc/free
> > > on bpf side.
> > >
> >
> > It seems that we'd better introduce ->memory_footprint for some
> > specific bpf maps. I will think about it.
>
> No. Don't build it into a replica of what we had before.
> Making existing bpf_map_memory_footprint() more accurate.
>

I just don't want to add many if-elses or switch-cases into
bpf_map_memory_footprint(), because I think it is a little ugly.
Introducing a new map ops could make it more clear.  For example,
static unsigned long bpf_map_memory_footprint(const struct bpf_map *map)
{
    unsigned long size;

    if (map->ops->map_mem_footprint)
        return map->ops->map_mem_footprint(map);

    size = round_up(map->key_size + bpf_map_value_size(map), 8);
    return round_up(map->max_entries * size, PAGE_SIZE);
}

> > > bpf side tracks all of its allocation. There is no need to do that
> > > in generic mm side.
> > > Exposing an aggregated single number if /proc/meminfo also looks wrong.
> >
> > Do you mean that we shouldn't expose it in /proc/meminfo ?
>
> We should not because it helps one particular use case only.
> Somebody else might want map mem info per container,
> then somebody would need it per user, etc.

It seems we should show memcg info and user info in bpftool map show.

> bpftool map show | awk
> solves all those cases without adding new uapi-s.

Makes sense to me.

-- 
Regards
Yafang