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

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Date:   Tue, 9 Jul 2019 21:26:58 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        Kairui Song <kasong@...hat.com>,
        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 07/09/2019 09:17 PM, Josh Poimboeuf wrote:
> On Tue, Jul 09, 2019 at 11:02:40AM -0700, Alexei Starovoitov wrote:
>> 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.

Same preference from my side.

>> 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.
> 
> Thanks!  I''ll probably send it through the tip tree, along with an
> objtool fix for the other optimization.

Ok, sounds good, thanks!