From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
To: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc: "Puranjay Mohan" <puranjay@kernel.org>,
"Björn Töpel" <bjorn@kernel.org>,
"Alexei Starovoitov" <ast@kernel.org>,
"Daniel Borkmann" <daniel@iogearbox.net>,
"Andrii Nakryiko" <andrii@kernel.org>,
"Martin KaFai Lau" <martin.lau@linux.dev>,
"Song Liu" <song@kernel.org>,
"Yonghong Song" <yonghong.song@linux.dev>,
"John Fastabend" <john.fastabend@gmail.com>,
"Kumar Kartikeya Dwivedi" <memxor@gmail.com>,
bpf <bpf@vger.kernel.org>
Subject: Re: On inlining more helpers in the JITs or the verifier
Date: Fri, 3 May 2024 09:04:49 -0700 [thread overview]
Message-ID: <CAADnVQ+rLCPBKNgSerzS8DKbdHEDgDyqUAjOYA-rP36oXwnY7A@mail.gmail.com> (raw)
In-Reply-To: <CAEf4BzYPQ7QS+UUxYj=okGyKirkqkmT+48DiRQFZ_DeC+zo+cA@mail.gmail.com>
On Thu, May 2, 2024 at 2:22 PM Andrii Nakryiko
<andrii.nakryiko@gmail.com> wrote:
>
> On Thu, May 2, 2024 at 10:37 AM Puranjay Mohan <puranjay@kernel.org> wrote:
> >
> >
> > Hi Everyone,
> >
> > While working on inlining bpf_get_smp_processor_id() in the ARM64 and
> > RISCV JITs, I realized that these archs allow such optimizations because
> > they keep some information like the per-cpu offset or the pointer to the
> > task_struct in special system registers.
> >
> > So, I went through the list of all BPF helpers and made a list of
> > helpers that we can inline in these JITs to make their usage much more
> > optimized:
> >
> > I. ARM64 and RISC-V specific optimzations if inlined:
> >
> > A) Because pointer to tast_struct is available in a register:
> > 1. bpf_get_current_pid_tgid()
> > 2. bpf_get_current_task()
>
> These two are used really frequently, so it might make sense to
> optimize them (and also bpf_get_current_task_btf(), of course), if
> others agree with me.
>
> > 3. bpf_set_retval()
> > 4. bpf_get_retval()
> > 5. bpf_task_pt_regs()
>
> I'm leaning towards saying that probably not, unless we have a really
> good reason to. Inlining is not free in terms of code maintenance and
> complexity, so I wouldn't go and inline everything possible. But maybe
> others have another opinion.
>
>
> > 6. bpf_get_attach_cookie()
>
> definitely no, there are multiple implementations depending on
> specific program type
>
> >
> > B) Because per_cpu offset is available in a register:
> > 1. bpf_this_cpu_ptr()
>
> maybe, but I don't think we inline at BPF instruction level, so
> inlining in BPF JIT seems premature
>
>
> > 2. bpf_get_numa_node_id()
>
> I'm not sure how actively this is used, so I'd say no to this one as well.
>
> >
> > These can be inlined in the verifier too using the newly
> > introduced per-cpu instruction.
>
> yep, I'd start with doing BPF assembly inlining for
> bpf_this_cpu_ptr/bpf_per_cpu_ptr, tbh
>
> >
> > II. These are very basic writes, can be inlined in the verifier or the JIT:
> > 1. bpf_msg_apply_bytes()
> > 2. bpf_msg_cork_bytes()
> > 3. bpf_set_hash_invalid()
>
> I'd say this is also going overboard with inlining.
+1
simplicity of logic is not a reason to inline it.
I would only inline bpf_get_current_task[_btf]() and do it
in the verifier. JITs should inline only if perf delta
is really significant.
I hope bpf_get_smp_processor_id() will be the only such example.
prev parent reply other threads:[~2024-05-03 16:05 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-05-02 17:37 On inlining more helpers in the JITs or the verifier Puranjay Mohan
2024-05-02 19:19 ` Puranjay Mohan
2024-05-02 21:22 ` Andrii Nakryiko
2024-05-03 16:04 ` Alexei Starovoitov [this message]
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