Re: [PATCH bpf-next v6 0/8] Transit between BPF TCP congestion controls.

[Kui-Feng Lee <sinquersw@gmail.com>]

On 3/9/23 20:38, Kui-Feng Lee wrote:
> Major changes:
> 
>   - Create bpf_links in the kernel for BPF struct_ops to register and
>     unregister it.
> 
>   - Enables switching between implementations of bpf-tcp-cc under a
>     name instantly by replacing the backing struct_ops map of a
>     bpf_link.
> 
> Previously, BPF struct_ops didn't go off, as even when the user
> program creating it was terminated, none of these ever were pinned.
> For instance, the TCP congestion control subsystem indirectly
> maintains a reference count on the struct_ops of any registered BPF
> implemented algorithm. Thus, the algorithm won't be deactivated until
> someone deliberately unregisters it.  For compatibility with other BPF
> programs, bpf_links have been created to work in coordination with
> struct_ops maps. This ensures that the registration and unregistration
> of these respective maps is carried out at the start and end of the
> bpf_link.
> 
> We also faced complications when attempting to replace an existing TCP
> congestion control algorithm with a new implementation on the fly. A
> struct_ops map was used to register a TCP congestion control algorithm
> with a unique name.  We had to either register the alternative
> implementation with a new name and move over or unregister the current
> one before being able to reregistration with the same name.  To fix
> this problem, we can an option to migrate the registration of the
> algorithm from struct_ops maps to bpf_links. By modifying the backing
> map of a bpf_link, it suddenly becomes possible to replace an existing
> TCP congestion control algorithm with ease.

The major differences from v5:

  - Add a new step to bpf_object__load() to prepare vdata.

  - Accept BPF_F_REPLACE.

  - Check section IDs in find_struct_ops_map_by_offset()

  - Add a test case to check mixing w/ & w/o link struct_ops.

  - Add a test case of using struct_ops w/o link to update a link.

  - Improve bpf_link__detach_struct_ops() to handle the w/ link case.


> 
> The major differences from v4:
> 
>   - Rebase.
> 
>   - Reorder patches and merge part 4 to part 2 of the v4.
> 
> The major differences from v3:
> 
>   - Remove bpf_struct_ops_map_free_rcu(), and use synchronize_rcu().
> 
>   - Improve the commit log of the part 1.
> 
>   - Before transitioning to the READY state, we conduct a value check
>     to ensure that struct_ops can be successfully utilized and links
>     created later.
> 
> The major differences from v2:
> 
>   - Simplify states
> 
>     - Remove TOBEUNREG.
> 
>     - Rename UNREG to READY.
> 
>   - Stop using the refcnt of the kvalue of a struct_ops. Explicitly
>     increase and decrease the refcount of struct_ops.
> 
>   - Prepare kernel vdata during the load phase of libbpf.
> 
> The major differences from v1:
> 
>   - Added bpf_struct_ops_link to replace the previous union-based
>     approach.
> 
>   - Added UNREG and TOBEUNREG to the state of bpf_struct_ops_map.
> 
>     - bpf_struct_ops_transit_state() maintains state transitions.
> 
>   - Fixed synchronization issue.
> 
>   - Prepare kernel vdata of struct_ops during the loading phase of
>     bpf_object.
> 
>   - Merged previous patch 3 to patch 1.
> 
v5: https://lore.kernel.org/all/20230308005050.255859-1-kuifeng@meta.com/
> v4: https://lore.kernel.org/all/20230307232913.576893-1-andrii@kernel.org/
> v3: https://lore.kernel.org/all/20230303012122.852654-1-kuifeng@meta.com/
> v2: https://lore.kernel.org/bpf/20230223011238.12313-1-kuifeng@meta.com/
> v1: https://lore.kernel.org/bpf/20230214221718.503964-1-kuifeng@meta.com/
> 
> Kui-Feng Lee (8):
>    bpf: Retire the struct_ops map kvalue->refcnt.
>    net: Update an existing TCP congestion control algorithm.
>    bpf: Create links for BPF struct_ops maps.
>    libbpf: Create a bpf_link in bpf_map__attach_struct_ops().
>    bpf: Update the struct_ops of a bpf_link.
>    libbpf: Update a bpf_link with another struct_ops.
>    libbpf: Use .struct_ops.link section to indicate a struct_ops with a
>      link.
>    selftests/bpf: Test switching TCP Congestion Control algorithms.
> 
>   include/linux/bpf.h                           |  10 +
>   include/net/tcp.h                             |   3 +
>   include/uapi/linux/bpf.h                      |  20 +-
>   kernel/bpf/bpf_struct_ops.c                   | 229 +++++++++++++++---
>   kernel/bpf/syscall.c                          |  49 +++-
>   net/bpf/bpf_dummy_struct_ops.c                |   6 +
>   net/ipv4/bpf_tcp_ca.c                         |  14 +-
>   net/ipv4/tcp_cong.c                           |  60 ++++-
>   tools/include/uapi/linux/bpf.h                |  20 +-
>   tools/lib/bpf/libbpf.c                        | 180 +++++++++++---
>   tools/lib/bpf/libbpf.h                        |   1 +
>   tools/lib/bpf/libbpf.map                      |   1 +
>   .../selftests/bpf/prog_tests/bpf_tcp_ca.c     |  91 +++++++
>   .../selftests/bpf/progs/tcp_ca_update.c       |  80 ++++++
>   14 files changed, 671 insertions(+), 93 deletions(-)
>   create mode 100644 tools/testing/selftests/bpf/progs/tcp_ca_update.c
>