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* [RFC bpf-next v1 0/4] Support cookie for link-based struct_ops attachment
@ 2025-07-08 23:08 Amery Hung
  2025-07-08 23:08 ` [RFC bpf-next v1 1/4] bpf: Factor out bpf_struct_ops_prepare_attach() Amery Hung
                   ` (3 more replies)
  0 siblings, 4 replies; 18+ messages in thread
From: Amery Hung @ 2025-07-08 23:08 UTC (permalink / raw)
  To: bpf
  Cc: alexei.starovoitov, andrii, daniel, tj, martin.lau, ameryhung,
	kernel-team

* Motivation *

Currently, sched_ext only supports a single scheduler. scx kfuncs can
reference the calling scheduler simply with a global pointer, scx_root,
whether being called in struct_ops, tracing, syscall or async callback.
However, when sched_ext moves to support multiple scx schedulers,
scx kfunc without task_struct argument won't be able to easily refer
to the calling scheduler without passing additional information.

The vision is to be able to view struct_ops, tracing, syscall programs
and async callbacks in a file as part of a scheduler, and have the
ability to refer to the scheduler in kfuncs called from any of the
programs.

However, right now there is not a notion that connects bpf programs in a
file in the kernel. Until it is clear what that should be, as a
stopgap, we would like to reuse cookie to associate bpf programs
belonging to the same scheduler.

* Design *

The interim solution is to use bpf cookie as the scheduler id, and add
the cookie as an argument to scx kfuncs without task_struct. By
maintaining the cookie to scheduler mapping in sched_ext, kfuncs will be
able to refer to the calling scheduler internally using cookie.

For the user space loader, it should use the same cookie when attaching
bpf programs. In bpf programs, they will call bpf_get_attach_cookie()
to get the cookie and pass it to scx kfuncs.

The following patches are the first part of the kernel-side plumbing: to
support cookie for link-based struct_ops attachment. struct_ops already
uses bpf trampoline. The only problem is the trampoline is being populated
during map_update when the cookie is not known yet. This patchset moves
the trampoline preparation from map_update to link_create for link-based
struct_ops attachment. Then, we extend the UAPI to take the cookie and
propagate it all the way to the struct_ops link_create. Finally,
bpf_get_attach_cookie() is allowed to struct_ops programs. 

A problem that has not been addressed in this RFC is that by moving
trampoline preparation to link_create, creating multiple links
using the same struct_ops map becomes problematic. A simple solution
may be just disable if there is no obvious use case for it.

* Alternative *

Martin has suggested a way that involves less kernel changes: Using
struct_ops map id as the sched_ext scheduler id. Instead of further
complicating struct_ops, sched_ext can create the struct_ops map id
to scheduler mapping in reg(). The id will also be stored in a global
variable in the file referenced by different types of programs. The
rest is similar to the cookie approach: kfuncs without task_struct
will need to add an argument, and they will lookup the scheduler by
the id.

* Previous approach *

The previous solution was to set a "this pointer" pointing to the
struct_ops struct during struct_ops map_update. Then, kfuncs can get
a pointer to the scheduler with a __prog argument. However, this
approach is broken as it would only work for struct_ops programs.


Some thought after Martin brought up the idea

I think we should use the map id instead of cookie. Especially if we are
going to disable creating multiple links per map, the cookie effectively
becomes the same as the map id.


Amery Hung (4):
  bpf: Factor out bpf_struct_ops_prepare_attach()
  bpf: Support cookie for linked-based struct_ops attachment
  libbpf: Support link-based struct_ops attach with options
  selftests/bpf: Test bpf_get_attach_cookie() in struct_ops program

 include/uapi/linux/bpf.h                      |   3 +
 kernel/bpf/bpf_struct_ops.c                   | 199 ++++++++++++------
 kernel/bpf/helpers.c                          |  18 ++
 tools/include/uapi/linux/bpf.h                |   3 +
 tools/lib/bpf/bpf.c                           |   5 +
 tools/lib/bpf/bpf.h                           |   3 +
 tools/lib/bpf/libbpf.c                        |  14 +-
 tools/lib/bpf/libbpf.h                        |  10 +
 tools/lib/bpf/libbpf.map                      |   1 +
 .../bpf/prog_tests/test_struct_ops_cookie.c   |  42 ++++
 .../selftests/bpf/progs/struct_ops_cookie.c   |  48 +++++
 .../selftests/bpf/test_kmods/bpf_testmod.c    |   1 +
 12 files changed, 280 insertions(+), 67 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/test_struct_ops_cookie.c
 create mode 100644 tools/testing/selftests/bpf/progs/struct_ops_cookie.c

-- 
2.47.1


^ permalink raw reply	[flat|nested] 18+ messages in thread
end of thread, other threads:[~2025-07-14 22:51 UTC | newest]

Thread overview: 18+ messages (download: mbox.gz follow: Atom feed
-- links below jump to the message on this page --
2025-07-08 23:08 [RFC bpf-next v1 0/4] Support cookie for link-based struct_ops attachment Amery Hung
2025-07-08 23:08 ` [RFC bpf-next v1 1/4] bpf: Factor out bpf_struct_ops_prepare_attach() Amery Hung
2025-07-08 23:08 ` [RFC bpf-next v1 2/4] bpf: Support cookie for linked-based struct_ops attachment Amery Hung
2025-07-09 22:13   ` Martin KaFai Lau
2025-07-10 18:26     ` Martin KaFai Lau
2025-07-10 18:39     ` Amery Hung
2025-07-10 19:47       ` Martin KaFai Lau
2025-07-10 21:00         ` Amery Hung
2025-07-11 18:41           ` Andrii Nakryiko
2025-07-11 19:29             ` Amery Hung
2025-07-11 20:21               ` Alexei Starovoitov
2025-07-11 21:38                 ` Amery Hung
2025-07-14 20:46                   ` Andrii Nakryiko
2025-07-14 21:02                     ` Amery Hung
2025-07-14 22:51                       ` Martin KaFai Lau
2025-07-11 21:55                 ` Martin KaFai Lau
2025-07-08 23:08 ` [RFC bpf-next v1 3/4] libbpf: Support link-based struct_ops attach with options Amery Hung
2025-07-08 23:08 ` [RFC bpf-next v1 4/4] selftests/bpf: Test bpf_get_attach_cookie() in struct_ops program Amery Hung

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