linux-kernel - Re: [RFC] bpf: Implement prealloc for task_local

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Message-ID: <CAEf4BzaSC7gWJSX+s0eudy=AMm_KHd3V92Ps4YMWGOmj-VOG4g@mail.gmail.com>
Date:   Fri, 22 Oct 2021 10:01:34 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Tejun Heo <tj@...nel.org>, Song Liu <songliubraving@...com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        bpf <bpf@...r.kernel.org>, Kernel Team <kernel-team@...com>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] bpf: Implement prealloc for task_local_storage

On Wed, Oct 20, 2021 at 1:16 PM Tejun Heo <tj@...nel.org> wrote:
>
> task_local_storage currently does not support pre-allocation and the memory
> is allocated on demand using the GFP_ATOMIC mask. While atomic allocations
> succeed most of the time and the occasional failures aren't a problem for
> many use-cases, there are some which can benefit from reliable allocations -
> e.g. tracking acquisitions and releases of specific resources to root cause
> long-term reference leaks.
>
> This patchset implements prealloc support for task_local_storage so that
> once a map is created, it's guaranteed that the storage area is always
> available for all tasks unless explicitly deleted.

Song, Martin, can you please take a look at this? It might be
worthwhile to consider having pre-allocated local storage for all
supported types: socket, cgroup, task. Especially for cases where BPF
app is going to touch all or almost all system entities (sockets,
cgroups, tasks, respectively).

Song, in ced47e30ab8b ("bpf: runqslower: Use task local storage") you
did some benchmarking of task-local storage vs hashmap and it was
faster in all cases but the first allocation of task-local storage. It
would be curious to see how numbers change if task-local storage is
pre-allocated, if you get a chance to benchmark it with Tejun's
changes. Thanks!

>
> The only tricky part is syncronizing against the fork path. Fortunately,
> cgroup needs to do the same thing and is already using
> cgroup_threadgroup_rwsem and we can use the same mechanism without adding
> extra overhead. This patchset generalizes the rwsem and make cgroup and bpf
> select it.
>
> This patchset is on top of bpf-next 223f903e9c83 ("bpf: Rename BTF_KIND_TAG
> to BTF_KIND_DECL_TAG") and contains the following patches:
>
>  0001-cgroup-Drop-cgroup_-prefix-from-cgroup_threadgroup_r.patch
>  0002-sched-cgroup-Generalize-threadgroup_rwsem.patch
>  0003-bpf-Implement-prealloc-for-task_local_storage.patch
>
> and also available in the following git branch:
>
>  git://git.kernel.org/pub/scm/linux/kernel/git/tj/misc.git bpf-task-local-storage-prealloc
>
> diffstat follows. Thanks.
>
>  fs/exec.c                                                   |    7 +
>  include/linux/bpf.h                                         |    6 +
>  include/linux/bpf_local_storage.h                           |   12 +++
>  include/linux/cgroup-defs.h                                 |   33 ---------
>  include/linux/cgroup.h                                      |   11 +--
>  include/linux/sched/threadgroup_rwsem.h                     |   46 ++++++++++++
>  init/Kconfig                                                |    4 +
>  kernel/bpf/Kconfig                                          |    1
>  kernel/bpf/bpf_local_storage.c                              |  112 ++++++++++++++++++++++--------
>  kernel/bpf/bpf_task_storage.c                               |  138 +++++++++++++++++++++++++++++++++++++-
>  kernel/cgroup/cgroup-internal.h                             |    4 -
>  kernel/cgroup/cgroup-v1.c                                   |    9 +-
>  kernel/cgroup/cgroup.c                                      |   74 ++++++++++++--------
>  kernel/cgroup/pids.c                                        |    2
>  kernel/fork.c                                               |   16 ++++
>  kernel/sched/core.c                                         |    4 +
>  kernel/sched/sched.h                                        |    1
>  kernel/signal.c                                             |    7 +
>  tools/testing/selftests/bpf/prog_tests/task_local_storage.c |  101 +++++++++++++++++++++++++++
>  tools/testing/selftests/bpf/progs/task_ls_prealloc.c        |   15 ++++
>  20 files changed, 489 insertions(+), 114 deletions(-)
>
> --
> tejun
>