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

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Date:   Tue, 9 Jul 2019 12:47:44 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, Song Liu <songliubraving@...com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        Kairui Song <kasong@...hat.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-tip-commits@...r.kernel.org
Subject: Re: [tip:x86/urgent] bpf: Fix ORC unwinding in non-JIT BPF code

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.

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)
 {
 #define BPF_INSN_2_LBL(x, y)    [BPF_##x | BPF_##y] = &&x##_##y
 #define BPF_INSN_3_LBL(x, y, z) [BPF_##x | BPF_##y | BPF_##z] = &&x##_##y##_##z