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

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On Fri, Jan 13, 2023 at 5:05 AM Alexei Starovoitov
<alexei.starovoitov@xxxxxxxxx> wrote:
>
> On Thu, Jan 12, 2023 at 7:53 AM Yafang Shao <laoar.shao@xxxxxxxxx> 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.

> 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.

> Higher level point:

Thanks for your thoughts.

> 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 ?

> People should be able to "bpftool map show|awk sum of fields"
> and get the same number.


-- 
Regards
Yafang



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