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
