Re: [PATCH v2 13/16] perf parse-events: Improvements to modifier parsing

[Liang, Kan]

On 2024-04-24 11:18 a.m., Ian Rogers wrote:
> On Fri, Apr 19, 2024 at 6:20 AM Liang, Kan <kan.liang@xxxxxxxxxxxxxxx> wrote:
>>
>>
>>
>> On 2024-04-19 2:22 a.m., Ian Rogers wrote:
>>>>> +             /* Simple modifiers copied to the evsel. */
>>>>> +             if (mod.precise) {
>>>>> +                     u8 precise = evsel->core.attr.precise_ip + mod.precise;
>>>>> +                     /*
>>>>> +                      * precise ip:
>>>>> +                      *
>>>>> +                      *  0 - SAMPLE_IP can have arbitrary skid
>>>>> +                      *  1 - SAMPLE_IP must have constant skid
>>>>> +                      *  2 - SAMPLE_IP requested to have 0 skid
>>>>> +                      *  3 - SAMPLE_IP must have 0 skid
>>>>> +                      *
>>>>> +                      *  See also PERF_RECORD_MISC_EXACT_IP
>>>>> +                      */
>>>>> +                     if (precise > 3) {
>>>> The pmu_max_precise() should return the max precise the current kernel
>>>> supports. It checks the /sys/devices/cpu/caps/max_precise.
>>>>
>>>> I think we should use that value rather than hard code it to 3.
>>> I'll add an extra patch to do that. I'm a bit concerned it may break
>>> event parsing on platforms not supporting max_precise of 3.
>>
>> The kernel already rejects the precise_ip > max_precise (using the same
>> x86_pmu_max_precise()). It should be fine to apply the same logic in the
>> tool.
>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/x86/events/core.c#n566
>>
>> Will the extra patch be sent separately?
> 
> Let's do it separately. I'm concerned about the behavior on AMD (and
> possibly similar architectures) where certain events support precision
> like cycles, as they detour to the IBS PMU, but not all events support
> it. The max_precise should reflect that AMD's Zen core PMU does
> support precision as a consequence of detouring to IBS, but maybe
> things in sysfs aren't set up correctly.
> 

The x86_pmu_max_precise() is a generic function for X86. It should apply
to AMD as well.

A separate patch looks good to me.

Thanks,
Kan