Re: [PATCH bpf-next 1/3] bpf: implement relay map basis

[Philo Lu <lulie@linux.alibaba.com>]

On 2023/12/23 21:02, Philo Lu wrote:
> 
> 
> On 2023/12/23 19:22, Hou Tao wrote:
>> Hi,
>>
>> On 12/22/2023 8:21 PM, Philo Lu wrote:
>>> BPF_MAP_TYPE_RELAY is implemented based on relay interface, which
>>> creates per-cpu buffer to transfer data. Each buffer is essentially a
>>> list of fix-sized sub-buffers, and is exposed to user space as files in
>>> debugfs. attr->max_entries is used as subbuf size and attr->map_extra is
>>> used as subbuf num. Currently, the default value of subbuf num is 8.
>>>
>>> The data can be accessed by read or mmap via these files. For example,
>>> if there are 2 cpus, files could be `/sys/kernel/debug/mydir/my_rmap0`
>>> and `/sys/kernel/debug/mydir/my_rmap1`.
>>>
>>> Buffer-only mode is used to create the relay map, which just allocates
>>> the buffer without creating user-space files. Then user can setup the
>>> files with map_update_elem, thus allowing user to define the directory
>>> name in debugfs. map_update_elem is implemented in the following patch.
>>>
>>> A new map flag named BPF_F_OVERWRITE is introduced to set overwrite mode
>>> of relay map.
>>
>> Beside adding a new map type, could we consider only use kfuncs to
>> support the creation of rchan and the write of rchan ? I think
>> bpf_cpumask will be a good reference.
> 
> This is a good question. TBH, I have thought of implement it with 
> helpers (I'm not very familiar with kfuncs, but I think they could be 
> similar?), but I was stumped by how to close the channel. We can create 
> a relay channel, hold it with a map, but it could be difficult for the 
> bpf program to close the channel with relay_close(). And I think it 
> could be the difference compared with bpf_cpumask.

I've learned more about kfunc and kptr, and find that kptr can be 
automatically released with a given map. Then, it is technically 
feasible to use relay interface with kfuncs. Specificially, creating a 
relay channel and getting the pointer with kfunc, transferring it as a 
kptr into a map, and then it lives with the map.

Though I'm not sure if this is better than map-based implementation, as 
mostly it will be used with a map (I haven't thought of a case without a 
map yet). And with kfunc, it will be a little more complex to create a 
relay channel.

Thanks.