linux-kernel - Re: [tip:x86/urgent] bpf: Fix ORC unwinding in non-JIT BPF code

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Date:   Tue, 9 Jul 2019 11:02:40 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        Kairui Song <kasong@...hat.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>, bpf <bpf@...r.kernel.org>
Subject: Re: [tip:x86/urgent] bpf: Fix ORC unwinding in non-JIT BPF code

On Tue, Jul 9, 2019 at 10:48 AM Josh Poimboeuf <jpoimboe@...hat.com> wrote:
>
> On Mon, Jul 08, 2019 at 04:16:25PM -0700, Alexei Starovoitov wrote:
> > total time is hard to compare.
> > Could you compare few tests?
> > like two that are called "tcpdump *"
> >
> > I think small regression is ok.
> > Folks that care about performance should be using JIT.
>
> I did each test 20 times and computed the averages:
>
> "tcpdump port 22":
>  default:       0.00743175s
>  -fno-gcse:     0.00709920s (~4.5% speedup)
>
> "tcpdump complex":
>  default:       0.00876715s
>  -fno-gcse:     0.00854895s (~2.5% speedup)
>
> So there does seem to be a small performance gain by disabling this
> optimization.

great. thanks for checking.

> We could change it for the whole file, by adjusting CFLAGS_core.o in the
> BPF makefile, or we could change it for the function only with something
> like the below patch.
>
> Thoughts?
>
> diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
> index e8579412ad21..d7ee4c6bad48 100644
> --- a/include/linux/compiler-gcc.h
> +++ b/include/linux/compiler-gcc.h
> @@ -170,3 +170,5 @@
>  #else
>  #define __diag_GCC_8(s)
>  #endif
> +
> +#define __no_fgcse __attribute__((optimize("-fno-gcse")))
> diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
> index 095d55c3834d..599c27b56c29 100644
> --- a/include/linux/compiler_types.h
> +++ b/include/linux/compiler_types.h
> @@ -189,6 +189,10 @@ struct ftrace_likely_data {
>  #define asm_volatile_goto(x...) asm goto(x)
>  #endif
>
> +#ifndef __no_fgcse
> +# define __no_fgcse
> +#endif
> +
>  /* Are two types/vars the same type (ignoring qualifiers)? */
>  #define __same_type(a, b) __builtin_types_compatible_p(typeof(a), typeof(b))
>
> diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> index 7e98f36a14e2..8191a7db2777 100644
> --- a/kernel/bpf/core.c
> +++ b/kernel/bpf/core.c
> @@ -1295,7 +1295,7 @@ bool bpf_opcode_in_insntable(u8 code)
>   *
>   * Decode and execute eBPF instructions.
>   */
> -static u64 ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn, u64 *stack)
> +static u64 __no_fgcse ___bpf_prog_run(u64 *regs, const struct bpf_insn *insn, u64 *stack)

I prefer per-function flag.
If you want to route it via tip:
Acked-by: Alexei Starovoitov <ast@...rnel.org>

or Daniel can take it into bpf tree while I'm traveling.