Re: [bpf-next 0/2] bpf: Add flag for batch operation

[dev@der-flo.net]

On Mon, Nov 11, 2024 at 07:01:26PM -0800, Alexei Starovoitov wrote:
> On Mon, Nov 11, 2024 at 6:15 AM Hou Tao <houtao@huaweicloud.com> wrote:
> >
> >
> >
> > On 11/10/2024 7:29 PM, Florian Lehner wrote:
> > > Introduce a new flag for batch operations that allows the deletion process
> > > to continue even if certain keys are missing. This simplifies map flushing
> > > by eliminating the requirement to maintain a separate list of keys and
> > > makes sure maps can be flushed with a single batch delete operation.
> >
> > Is it expensive to close and recreate a new map instead ? If it is
> > expensive, does it make more sense to add a new command to delete all
> > elements in the map ? Because reusing the deletion logic will make each
> > deletion involve an unnecessary lookup operation.
> 
> +1 to above questions.

There is an eBPF map, that a variable number of eBPF programs use, to access
common states for a variable number of connections. On predefined events, a set
of keys is deleted from this map. This set can either be all keys or just a
subset of all keys - but it is not guaranteed that this set of keys still exists
in this eBPF map.
The current work around is to use bpf_map_lookup_and_delete_batch(), as this
operation continues on missing keys and clears all requested keys from the eBPF
map. The noticeable downside of bpf_map_lookup_and_delete_batch() is the memory
requirement that comes with the lookup and allocation for the values.

> > [..] If it is
> > expensive, does it make more sense to add a new command to delete all
> > elements in the map ?

It felt like bpf_map_delete_batch() was introduced for this use case. So adding
a new command was not considered.

> 
> In addition:
> 
> What is the use case ?
> Are you trying to erase all elements from the map ?
> 
> If so you bpf_for_each_map_elem() and delete elems while iterating.

bpf_for_each_map_elem() could be an option if the map should be flushed
completley, but in most cases only a subset of keys should be removed from the
map.

> 
> This extra flag looks too specific.

Sure, the proposed flag is focused on the delete operation. What could be the
requirement to make it less specific?

> 
> pw-bot: cr