Re: [PATCH bpf-next 7/7] bpf: hashtab memory usage

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

On Sat, Feb 4, 2023 at 7:56 PM Yafang Shao <laoar.shao@gmail.com> wrote:
>
> On Sat, Feb 4, 2023 at 10:01 AM John Fastabend <john.fastabend@gmail.com> wrote:
> >
> > Yafang Shao wrote:
> > > Get htab memory usage from the htab pointers we have allocated. Some
> > > small pointers are ignored as their size are quite small compared with
> > > the total size.
> > >
> > > The result as follows,
> > > - before this change
> > > 1: hash  name count_map  flags 0x0  <<<< prealloc
> > >         key 16B  value 24B  max_entries 1048576  memlock 41943040B
> > > 2: hash  name count_map  flags 0x1  <<<< non prealloc, fully set
> > >         key 16B  value 24B  max_entries 1048576  memlock 41943040B
> > > 3: hash  name count_map  flags 0x1  <<<< non prealloc, non set
> > >         key 16B  value 24B  max_entries 1048576  memlock 41943040B
> > >
> > > The memlock is always a fixed number whatever it is preallocated or
> > > not, and whatever the allocated elements number is.
> > >
> > > - after this change
> > > 1: hash  name count_map  flags 0x0  <<<< prealloc
> > >         key 16B  value 24B  max_entries 1048576  memlock 109064464B
> > > 2: hash  name count_map  flags 0x1  <<<< non prealloc, fully set
> > >         key 16B  value 24B  max_entries 1048576  memlock 117464320B
> > > 3: hash  name count_map  flags 0x1  <<<< non prealloc, non set
> > >         key 16B  value 24B  max_entries 1048576  memlock 16797952B
> > >
> > > The memlock now is hashtab actually allocated.
> > >
> > > At worst, the difference can be 10x, for example,
> > > - before this change
> > > 4: hash  name count_map  flags 0x0
> > >         key 4B  value 4B  max_entries 1048576  memlock 8388608B
> > >
> > > - after this change
> > > 4: hash  name count_map  flags 0x0
> > >         key 4B  value 4B  max_entries 1048576  memlock 83898640B
> > >
> >
> > This walks the entire map and buckets to get the size? Inside a
> > rcu critical section as well :/ it seems.
> >
>
> No, it doesn't walk the entire map and buckets, but just gets one
> random element.
> So it won't be a problem to do it inside a rcu critical section.
>
> > What am I missing, if you know how many elements are added (which
> > you can track on map updates) how come we can't just calculate the
> > memory size directly from this?
> >
>
> It is less accurate and hard to understand. Take non-preallocated
> percpu hashtab for example,
> The size can be calculated as follows,
>     key_size = round_up(htab->map.key_size, 8)；
>     value_size = round_up(htab->map.value_size, 8);
>     pcpu_meta_size = sizeof(struct llist_node) + sizeof(void *);
>     usage = ((value_size * num_possible_cpus() +\
>                     pcpu_meta_size + key_size) * max_entries
>
> That is quite unfriendly to the newbies, and may be error-prone.

Please do that instead.
map_mem_usage callback is a no go as I mentioned earlier.

> Furthermore, it is less accurate because there is underlying memory
> allocation in the MM subsystem.
> Now we can get a more accurate usage with little overhead. Why not do it?

because htab_mem_usage() is not maintainable long term.
100% accuracy is a non-goal.