netdev - Re: [PATCH bpf-next 1/3] bpf: add bpf queue map

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Date:   Thu, 9 Aug 2018 11:02:51 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Mauricio Vasquez <mauricio.vasquez@...ito.it>
Cc:     Alexei Starovoitov <ast@...nel.org>, netdev@...r.kernel.org
Subject: Re: [PATCH bpf-next 1/3] bpf: add bpf queue map

On 08/09/2018 06:48 AM, Alexei Starovoitov wrote:
> On Wed, Aug 08, 2018 at 10:08:47PM -0500, Mauricio Vasquez wrote:
>>
>>> And how about adding three new helpers: push/pop/peek as well?
>>> Reusing lookup/update is neat, but does lookup == pop
>>> or does lookup == peek ?
>>> I suspect it will be confusing.
>>> Three new helpers cost nothing, but will make bpf progs easier to read.
>> I agree. I have one doubt here, update/lookup/delete is implemented in all
>> map types, if the operation is not supported it returns -EINVAL.
>> For push/pop/peek, should we implement it in all maps or is it better to
>> check the map type before invoking map->ops->push/pop/seek?
>> (Maybe checking if map->ops->xxx is NULL will also work)
> 
> Since push/pop/peak are only for this queue/stack I thought we won't
> be adding 'ops' for them and just call the helpers from progs.
> But now I'm having second thoughts, since 3 new commands for syscall
> feels like overkill.
> At the same time I still don't feel that lookup == pop is the right alias.
> Also what peak is going to alias to ?
> May be let's go back to your original idea with a tweak:
> push == update
> peak == lookup
> pop = lookup + delete
> In other words in case of stack the bpf_map_lookup_elem will return
> the pointer to value of top element in the stack and
> bpf_map_delete_elem will delete that top element.
> Then in user space we can introduce push/pop always_inline functions like:
> void bpf_push(struct bpf_map *map, void *value)
> {
>   bpf_map_update_elem(map, NULL/*key*/, value);
> }
> 
> void *bpf_pop(struct bpf_map *map)
> {
>   void * val = bpf_map_lookup_elem(map, NULL/*key*/);
>   bpf_map_delete_elem(map, NULL/*key*/);
>   return val;
> }
> 
> Thoughts?

I actually think that having new push/peak/pop BPF map helpers would
be fine, as well as having them sit in map->ops. Initially I'd probably
leave out the syscall ops counterparts so they can be used only from
program.

Potentially array and per-cpu array could implement them even by having
an internal pointer to the current slot which we move on push/pop. This
would potentially also avoid all the RCU callbacks to free the elements,
elem allocations, list locking, and improve cache locality compared to
the current implementation.

Agree that existing ops are not the right alias, but deferring to user
space as inline function also doesn't really seem like a good fit, imho,
so I'd prefer rather to have something native. (Aside from that, the
above inline bpf_pop() would also race between CPUs.)

Thanks,
Daniel