[bpf-next,v2,0/2] Introduce bpf_preempt_{disable,enable}

Message

Kumar Kartikeya Dwivedi April 24, 2024, 3:13 a.m. UTC

This set introduces two kfuncs, bpf_preempt_disable and
bpf_preempt_enable, which are wrappers around preempt_disable and
preempt_enable in the kernel. These functions allow a BPF program to
have code sections where preemption is disabled. There are multiple use
cases that are served by such a feature, a few are listed below:

1. Writing safe per-CPU alogrithms/data structures that work correctly
   across different contexts.
2. Writing safe per-CPU allocators similar to bpf_memalloc on top of
   array/arena memory blobs.
3. Writing locking algorithms in BPF programs natively.

Note that local_irq_disable/enable equivalent is also needed for proper
IRQ context protection, but that is a more involved change and will be
sent later.

While bpf_preempt_{disable,enable} is not sufficient for all of these
usage scenarios on its own, it is still necessary.

The same effect as these kfuncs can in some sense be already achieved
using the bpf_spin_lock or rcu_read_lock APIs, therefore from the
standpoint of kernel functionality exposure in the verifier, this is
well understood territory.

Note that these helpers do allow calling kernel helpers and kfuncs from
within the non-preemptible region (unless sleepable). Otherwise, any
locks built using the preemption helpers will be as limited as
existing bpf_spin_lock.

Nesting is allowed by keeping a counter for tracking remaining enables
required to be performed. Similar approach can be applied to
rcu_read_locks in a follow up.

Changelog
=========
v1: https://lore.kernel.org/bpf/[e-mail]

 * Move kfunc BTF ID declerations above css task kfunc for
   !CONFIG_CGROUPS config (Alexei)
 * Add test case for global function call in non-preemptible region
   (Jiri)

Kumar Kartikeya Dwivedi (2):
  bpf: Introduce bpf_preempt_[disable,enable] kfuncs
  selftests/bpf: Add tests for preempt kfuncs

 include/linux/bpf_verifier.h                  |   1 +
 kernel/bpf/helpers.c                          |  12 ++
 kernel/bpf/verifier.c                         |  71 ++++++++-
 .../selftests/bpf/prog_tests/preempt_lock.c   |   9 ++
 .../selftests/bpf/progs/preempt_lock.c        | 135 ++++++++++++++++++
 5 files changed, 226 insertions(+), 2 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/preempt_lock.c
 create mode 100644 tools/testing/selftests/bpf/progs/preempt_lock.c


base-commit: 6e10b6350a67d398c795ac0b93a7bb7103633fe4

Comments

Jiri Olsa April 24, 2024, 11:06 a.m. UTC | #1

On Wed, Apr 24, 2024 at 03:13:13AM +0000, Kumar Kartikeya Dwivedi wrote:
> This set introduces two kfuncs, bpf_preempt_disable and
> bpf_preempt_enable, which are wrappers around preempt_disable and
> preempt_enable in the kernel. These functions allow a BPF program to
> have code sections where preemption is disabled. There are multiple use
> cases that are served by such a feature, a few are listed below:
> 
> 1. Writing safe per-CPU alogrithms/data structures that work correctly
>    across different contexts.
> 2. Writing safe per-CPU allocators similar to bpf_memalloc on top of
>    array/arena memory blobs.
> 3. Writing locking algorithms in BPF programs natively.
> 
> Note that local_irq_disable/enable equivalent is also needed for proper
> IRQ context protection, but that is a more involved change and will be
> sent later.
> 
> While bpf_preempt_{disable,enable} is not sufficient for all of these
> usage scenarios on its own, it is still necessary.
> 
> The same effect as these kfuncs can in some sense be already achieved
> using the bpf_spin_lock or rcu_read_lock APIs, therefore from the
> standpoint of kernel functionality exposure in the verifier, this is
> well understood territory.
> 
> Note that these helpers do allow calling kernel helpers and kfuncs from
> within the non-preemptible region (unless sleepable). Otherwise, any
> locks built using the preemption helpers will be as limited as
> existing bpf_spin_lock.
> 
> Nesting is allowed by keeping a counter for tracking remaining enables
> required to be performed. Similar approach can be applied to
> rcu_read_locks in a follow up.
> 
> Changelog
> =========
> v1: https://lore.kernel.org/bpf/[e-mail]
> 
>  * Move kfunc BTF ID declerations above css task kfunc for
>    !CONFIG_CGROUPS config (Alexei)
>  * Add test case for global function call in non-preemptible region
>    (Jiri)

Acked-by: Jiri Olsa <[e-mail]>

jirka

> 
> Kumar Kartikeya Dwivedi (2):
>   bpf: Introduce bpf_preempt_[disable,enable] kfuncs
>   selftests/bpf: Add tests for preempt kfuncs
> 
>  include/linux/bpf_verifier.h                  |   1 +
>  kernel/bpf/helpers.c                          |  12 ++
>  kernel/bpf/verifier.c                         |  71 ++++++++-
>  .../selftests/bpf/prog_tests/preempt_lock.c   |   9 ++
>  .../selftests/bpf/progs/preempt_lock.c        | 135 ++++++++++++++++++
>  5 files changed, 226 insertions(+), 2 deletions(-)
>  create mode 100644 tools/testing/selftests/bpf/prog_tests/preempt_lock.c
>  create mode 100644 tools/testing/selftests/bpf/progs/preempt_lock.c
> 
> 
> base-commit: 6e10b6350a67d398c795ac0b93a7bb7103633fe4
> -- 
> 2.43.0
> 
>

[e-mail] April 24, 2024, 5 p.m. UTC | #2

Hello:

This series was applied to bpf/bpf-next.git (master)
by Alexei Starovoitov <[e-mail]>:

On Wed, 24 Apr 2024 03:13:13 +0000 you wrote:
> This set introduces two kfuncs, bpf_preempt_disable and
> bpf_preempt_enable, which are wrappers around preempt_disable and
> preempt_enable in the kernel. These functions allow a BPF program to
> have code sections where preemption is disabled. There are multiple use
> cases that are served by such a feature, a few are listed below:
> 
> 1. Writing safe per-CPU alogrithms/data structures that work correctly
>    across different contexts.
> 2. Writing safe per-CPU allocators similar to bpf_memalloc on top of
>    array/arena memory blobs.
> 3. Writing locking algorithms in BPF programs natively.
> 
> [...]

Here is the summary with links:
  - [bpf-next,v2,1/2] bpf: Introduce bpf_preempt_[disable,enable] kfuncs
    https://git.kernel.org/bpf/bpf-next/c/fc7566ad0a82
  - [bpf-next,v2,2/2] selftests/bpf: Add tests for preempt kfuncs
    https://git.kernel.org/bpf/bpf-next/c/3134396f1cba

You are awesome, thank you!