Re: [PATCH] drivers/perf: arm_pmu: Show PMU version on boot

[Shaokun Zhang <zhangshaokun@hisilicon.com>]

Hi Will,

在 2020/9/7 21:24, Will Deacon 写道:
> On Wed, Sep 02, 2020 at 09:31:00AM +0800, Shaokun Zhang wrote:
>> 在 2020/9/2 0:16, Will Deacon 写道:
>>> On Wed, Aug 26, 2020 at 02:42:26PM +0800, Shaokun Zhang wrote:
>>>> 在 2020/8/22 0:03, Will Deacon 写道:
>>>>> On Thu, Jul 30, 2020 at 06:47:21PM +0800, Shaokun Zhang wrote:
>>>>>> The @pmuver field has been initialized and can tell the PMU version.
>>>>>> Let's show it on boot and the user obtains this information directly.
>>>>>>
>>>>>> Cc: Will Deacon <will@kernel.org>
>>>>>> Cc: Mark Rutland <mark.rutland@arm.com>
>>>>>> Signed-off-by: Shaokun Zhang <zhangshaokun@hisilicon.com>
>>>>>> ---
>>>>>>  drivers/perf/arm_pmu.c | 4 ++--
>>>>>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>>>>>
>>>>>> diff --git a/drivers/perf/arm_pmu.c b/drivers/perf/arm_pmu.c
>>>>>> index df352b334ea7..36f7fad7ba5a 100644
>>>>>> --- a/drivers/perf/arm_pmu.c
>>>>>> +++ b/drivers/perf/arm_pmu.c
>>>>>> @@ -870,8 +870,8 @@ int armpmu_register(struct arm_pmu *pmu)
>>>>>>  	if (!__oprofile_cpu_pmu)
>>>>>>  		__oprofile_cpu_pmu = pmu;
>>>>>>  
>>>>>> -	pr_info("enabled with %s PMU driver, %d counters available\n",
>>>>>> -		pmu->name, pmu->num_events);
>>>>>> +	pr_info("enabled with %s PMU driver, %d counters available, version is %d\n",
>>>>>> +		pmu->name, pmu->num_events, pmu->pmuver);
>>>>>
>>>>> Hmm. I'm suspicious about this. Who is using this, and what for? We're
>>>>
>>>> Since Arm ARM has extended PMU version for Armv8.1/8.4/8.5 with different PMUver in
>>>> ID_AA64DFR0_EL1, someone who does the performance profiling may care the number of
>>>> counter and want to know the supported PMU version. Because some events or features
>>>> are related to PMU version, Like events corresponding to PMCEID0/1_EL0[32:63] or if
>>>> anyone be told PMMIR_EL1 to ZERO, he can check the PMU version directly from the boot
>>>> log information.
>>>
>>> What do you mean by "may care the number of counter"? The number of counters
>>
>> If the user doesn't want the kernel to use time multiplexing, it shall use events less
>> than the counters that it has been told by the boot information, of course, he can access
>> PMCR_EL0 directly to check it.
> 
> But this isn't an arm64-specific problem, is it? What happens on other

Correct, but I don't mention it as a problem, I just want to explain why some user may check
the counter number information. Apologies that I gave some misunderstanding words.

> architectures, such as x86?
> 
>>> is independent of the PMU version and shouldn't really be of interest to
>>> userspace anyway, should it? Or are you referring to something else?
>>
>> Yes, it is independent, I mean that if the user is interested in PMU information, like,
>> PMU counter number, PMU version, etc, it can be exposed at boot time directly.
> 
> If userspace needs information about the PMU, I'm up for exposing it via
> sysfs, but I don't think we should expose things for the sake of it, and I
> also think that information which isn't specific to the particular PMU
> should be exposed more generally so that userspace doesn't need lots of
> different parsers to get the same information on different hardware.
> 

Got it, thanks for more explanations.

>>> As for whether events are supported or not, we already expose that on a
>>> per-event basis, and I think we should continue to do that.
>>
>> Shall we check the PMU version(ARMv8.1) in the driver for PMCEID0/1_EL0[32:63]?
> 
> I'd need to see the patch, since I don't see where you're going with this.

Oh，I'm not sure whether we shall check PMU verion for PMCEID0/1_EL0[32:63] since
these are ARMv8.1-PMU events. Indeed I didn't do it anything about it in this patch.

Thanks,
Shaokun

> 

> Will
> 
> .
> 


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