Re: [BUG] perf: sampling with precise=2 broken in 3.18

[Stephane Eranian]

On Tue, Dec 16, 2014 at 5:46 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Mon, Dec 15, 2014 at 11:51:07PM -0500, Stephane Eranian wrote:
>> Hi,
>>
>> I was running some perf mem test for an upcoming patch when
>> I realize that precise=2 was broken on 3.18. It seems it never
>> (or extremely rarely) correct the off-by-one error, when until 3.18-rc4
>> it was 100% on the same program. So something was introduced
>> that broke the asm walker in perf_event_intel_ds.c.
>>
>> Looking at the log of that file, I can see one change that could have
>> some impact:
>>
>> Author: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
>> 6ba48ff x86: Remove arbitrary instruction size limit in instruction decoder
>>
>> if I use a kernel without this fix (prior to that commit), then correction
>> works. Any kernel after fails. I have not investigated why but may you
>> have an idea.
>>
>> To reproduce try using perf mem -t load rec my_load_test, then use
>> perf report to navigate to the assembly view, the samples should be
>> on load instructions, not on the instructions following them. If you use
>> perf mem -t load rec -vv you can verify that precise=2. So something
>> is not working anymore in the instruction decoder that the fixup routine
>> bails out.
>>
>> Any clue?
>
> This appears to have fixed it.
>
> ---
> Subject: x86: Fix off-by-one in instruction decoder
>
> Stephane reported that the PEBS fixup was broken by the recent commit to
> the instruction decoder. The thing had an off-by-one which resulted in
> not being able to decode the last instruction and always bail.
>
Works again now. Thanks for fixing this quickly.

Acked-by: Stephane Eranian <eranian@xxxxxxxxxx>

> Reported-by: Stephane Eranian <eranian@xxxxxxxxxx>
> Fixes: 6ba48ff46f76 ("x86: Remove arbitrary instruction size limit in instruction decoder")
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> ---
>  arch/x86/lib/insn.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/lib/insn.c b/arch/x86/lib/insn.c
> index 2480978..1313ae6 100644
> --- a/arch/x86/lib/insn.c
> +++ b/arch/x86/lib/insn.c
> @@ -28,7 +28,7 @@
>
>  /* Verify next sizeof(t) bytes can be on the same instruction */
>  #define validate_next(t, insn, n)      \
> -       ((insn)->next_byte + sizeof(t) + n < (insn)->end_kaddr)
> +       ((insn)->next_byte + sizeof(t) + n <= (insn)->end_kaddr)
>
>  #define __get_next(t, insn)    \
>         ({ t r = *(t*)insn->next_byte; insn->next_byte += sizeof(t); r; })
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