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

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

On Wed, Feb 8, 2023 at 9:56 AM Alexei Starovoitov
<alexei.starovoitov@gmail.com> wrote:
>
> 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.

I can do it as you suggested, but it seems we shouldn't keep all
estimates in one place. Because ...

> map_mem_usage callback is a no go as I mentioned earlier.

...we have to introduce the map_mem_usage callback. Take the lpm_trie
for example, its usage is
usage = (sizeof(struct lpm_trie_node) + trie->data_size) * trie->n_entries;

I don't think we want  to declare struct lpm_trie_node in kernel/bpf/syscall.c.
WDYT ?

>
> > 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.

htab_mem_usage() gives us an option to continue to make it more
accurate with considerable overhead.
But I won't insist on it if you think we don't need to make it more accurate.

-- 
Regards
Yafang