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

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

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.

> > 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.

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