linux-kernel - Re: [PATCH] perf/x86/intel/lbr: fix branch type encoding

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Date:   Thu, 11 Aug 2022 10:41:51 -0400
From:   "Liang, Kan" <kan.liang@...ux.intel.com>
To:     Stephane Eranian <eranian@...gle.com>
Cc:     linux-kernel@...r.kernel.org, peterz@...radead.org,
        kan.liang@...el.com, ak@...ux.intel.com, acme@...hat.com,
        namhyung@...nel.org, irogers@...gle.com
Subject: Re: [PATCH] perf/x86/intel/lbr: fix branch type encoding



On 2022-08-11 10:17 a.m., Stephane Eranian wrote:
> On Thu, Aug 11, 2022 at 3:23 PM Liang, Kan <kan.liang@...ux.intel.com> wrote:
>>
>>
>>
>> On 2022-08-10 5:06 p.m., Stephane Eranian wrote:
>>> With architected LBR, the procesosr can record the type of each sampled taken
>>> branch. The type is encoded in 4-bit field in the LBR_INFO MSR of each entry.
>>>
>>> The branch type must then extracted and saved in the perf_branch_entry in the
>>> perf_events sampling buffer. With the current code, the raw Intel encoding of
>>> the branch is exported to user tools.
>>
>> In the intel_pmu_lbr_filter(), the raw encoding will be converted into
>> the X86_BR_* format via arch_lbr_br_type_map[]. Then the
>> common_branch_type() will convert the X86_BR_* format to the generic
>> PERF_BR_* type and expose to user tools.
>>
>> I double check the existing arch_lbr_br_type_map[] and branch_map[].
>> They should generate the same PERF_BR_* type as your arch_lbr_type_map[].
>>
>> Is there a test case which I can use to reproduce the problem?
>>
> I was doing a simple:
> $ perf record -b -e cpu/event=0xc4/ ....
> $ perf report -D
> Looking at the LBR information and the BR type, many entries has no branch type.
> What I see is a function where you do: e->type = get_lbr_br_type() and
> that is what
> is then saved in the buffer. Unless I am missing a later patch.
>

To get the LBR type, the save_type filter option must be applied. See
60f83fa6341d ("perf record: Create a new option save_type in
--branch-filter").

The -b only include the ANY option. Maybe we should extend the -b option
to ANY|SAVE_TYPE.

Thanks,
Kan

> 
>> Thanks,
>> Kan
>>
>>> Yet tools, such as perf, expected the
>>> branch type to be encoded using perf_events branch type enum
>>> (see tools/perf/util/branch.c). As a result of the discrepancy, the output of
>>> perf report -D shows bogus branch types.
>>>
>>> Fix the problem by converting the Intel raw encoding into the perf_events
>>> branch type enum values. With that in place and with no changes to the tools,
>>> the branch types are now reported properly.
>>>
>>> Signed-off-by: Stephane Eranian <eranian@...gle.com>
>>> ---
>>>  arch/x86/events/intel/lbr.c | 35 ++++++++++++++++++++++++++++++++---
>>>  1 file changed, 32 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/arch/x86/events/intel/lbr.c b/arch/x86/events/intel/lbr.c
>>> index 4f70fb6c2c1e..ef63d4d46b50 100644
>>> --- a/arch/x86/events/intel/lbr.c
>>> +++ b/arch/x86/events/intel/lbr.c
>>> @@ -894,9 +894,23 @@ static DEFINE_STATIC_KEY_FALSE(x86_lbr_mispred);
>>>  static DEFINE_STATIC_KEY_FALSE(x86_lbr_cycles);
>>>  static DEFINE_STATIC_KEY_FALSE(x86_lbr_type);
>>>
>>> -static __always_inline int get_lbr_br_type(u64 info)
>>> +/*
>>> + * Array index encodes IA32_LBR_x_INFO Branch Type Encodings
>>> + * as per Intel SDM Vol3b Branch Types section
>>> + */
>>> +static const int arch_lbr_type_map[]={
>>> +     [0] = PERF_BR_COND,
>>> +     [1] = PERF_BR_IND,
>>> +     [2] = PERF_BR_UNCOND,
>>> +     [3] = PERF_BR_IND_CALL,
>>> +     [4] = PERF_BR_CALL,
>>> +     [5] = PERF_BR_RET,
>>> +};
>>> +#define ARCH_LBR_TYPE_COUNT ARRAY_SIZE(arch_lbr_type_map)
>>> +
>>> +static __always_inline u16 get_lbr_br_type(u64 info)
>>>  {
>>> -     int type = 0;
>>> +     u16 type = 0;
>>>
>>>       if (static_branch_likely(&x86_lbr_type))
>>>               type = (info & LBR_INFO_BR_TYPE) >> LBR_INFO_BR_TYPE_OFFSET;
>>> @@ -904,6 +918,21 @@ static __always_inline int get_lbr_br_type(u64 info)
>>>       return type;
>>>  }
>>>
>>> +/*
>>> + * The kernel cannot expose raw Intel branch type encodings because they are
>>> + * not generic. Instead, the function below  maps the encoding to the
>>> + * perf_events user visible branch types.
>>> + */
>>> +static __always_inline int get_lbr_br_type_mapping(u64 info)
>>> +{
>>> +     if (static_branch_likely(&x86_lbr_type)) {
>>> +             u16 raw_type = get_lbr_br_type(info);
>>> +             if (raw_type < ARCH_LBR_TYPE_COUNT)
>>> +                     return arch_lbr_type_map[raw_type];
>>> +     }
>>> +     return PERF_BR_UNKNOWN;
>>> +}
>>> +
>>>  static __always_inline bool get_lbr_mispred(u64 info)
>>>  {
>>>       bool mispred = 0;
>>> @@ -957,7 +986,7 @@ static void intel_pmu_store_lbr(struct cpu_hw_events *cpuc,
>>>               e->in_tx        = !!(info & LBR_INFO_IN_TX);
>>>               e->abort        = !!(info & LBR_INFO_ABORT);
>>>               e->cycles       = get_lbr_cycles(info);
>>> -             e->type         = get_lbr_br_type(info);
>>> +             e->type         = get_lbr_br_type_mapping(info);
>>>       }
>>>
>>>       cpuc->lbr_stack.nr = i;