Re: [PATCH v4 3/4] perf/core: Remove pmu linear searching code

[Thomas Richter <tmricht@linux.ibm.com>]

On 5/27/23 20:38, Marc Zyngier wrote:
> On Sat, 27 May 2023 18:00:13 +0100,
> Ian Rogers <irogers@google.com> wrote:
>>
>> On Sat, May 27, 2023 at 6:32 AM Marc Zyngier <maz@kernel.org> wrote:
>>>
>>> On Sat, 27 May 2023 00:00:47 +0100,
>>> Ian Rogers <irogers@google.com> wrote:
>>>>
>>>> On Thu, May 25, 2023 at 8:56 AM Oliver Upton <oliver.upton@linux.dev> wrote:
>>>>>
>>>>> On Thu, May 25, 2023 at 04:20:31PM +0200, Peter Zijlstra wrote:
>>>>>> On Thu, May 25, 2023 at 07:11:41AM +0000, Oliver Upton wrote:
>>>>>>
>>>>>>> The PMUv3 driver does pass a name, but it relies on getting back an
>>>>>>> allocated pmu id as @type is -1 in the call to perf_pmu_register().
>>>>>>>
>>>>>>> What actually broke is how KVM probes for a default core PMU to use for
>>>>>>> a guest. kvm_pmu_probe_armpmu() creates a counter w/ PERF_TYPE_RAW and
>>>>>>> reads the pmu from the returned perf_event. The linear search had the
>>>>>>> effect of eventually stumbling on the correct core PMU and succeeding.
>>>>>>>
>>>>>>> Perf folks: is this WAI for heterogenous systems?
>>>>>>
>>>>>> TBH, I'm not sure. hetero and virt don't mix very well AFAIK and I'm not
>>>>>> sure what ARM64 does here.
>>>>>>
>>>>>> IIRC the only way is to hard affine things; that is, force vCPU of
>>>>>> 'type' to the pCPU mask of 'type' CPUs.
>>>>>
>>>>> We provide absolutely no illusion of consistency across implementations.
>>>>> Userspace can select the PMU type, and then it is a userspace problem
>>>>> affining vCPUs to the right pCPUs.
>>>>>
>>>>> And if they get that wrong, we just bail and refuse to run the vCPU.
>>>>>
>>>>>> If you don't do that; or let userspace 'override' that, things go
>>>>>> sideways *real* fast.
>>>>>
>>>>> Oh yeah, and I wish PMUs were the only problem with these hetero
>>>>> systems...
>>>>
>>>> Just to add some context from what I understand. There are inbuilt
>>>> type numbers for PMUs:
>>>> https://git.kernel.org/pub/scm/linux/kernel/git/acme/linux.git/tree/include/uapi/linux/perf_event.h?h=perf-tools-next#n34
>>>> so the PMU generally called /sys/devices/cpu should have type 4 (ARM
>>>> give it another name). For heterogeneous ARM there is a single PMU and
>>>> the same events are programmed regardless of whether it is a big or a
>>>> little core - the cpumask lists all CPUs.
>>>
>>> I think you misunderstood the way heterogeneous arm64 systems are
>>> described . Each CPU type gets its own PMU type, and its own event
>>> list. Case in point:
>>>
>>> $ grep . /sys/devices/*pmu/{type,cpus}
>>> /sys/devices/apple_avalanche_pmu/type:9
>>> /sys/devices/apple_blizzard_pmu/type:8
>>> /sys/devices/apple_avalanche_pmu/cpus:4-9
>>> /sys/devices/apple_blizzard_pmu/cpus:0-3
>>>
>>> Type 4 (aka PERF_EVENT_RAW) is AFAICT just a way to encode the raw
>>> event number, nothing else.
>>
>> Which PMU will a raw event open on?
> 
> On the PMU that matches the current CPU.
> 
>> Note, the raw events don't support
>> the extended type that is present in PERF_TYPE_HARDWARE and
>> PERF_TYPE_HW_CACHE:
>> https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git/tree/include/uapi/linux/perf_event.h#n41
>> as the bits are already in use for being just plain config values.
> 
> I'm not sure how relevant this is to the numbering of PMUs on arm64.
> 
>> I suspect not being type 4 is a bug on apple ARM here.
> 
> If that's a bug on this machine, it's a bug on all machines, at which
> point it is the de-facto API:
> 
> $ grep . /sys/devices/armv8*/{type,cpus}
> /sys/devices/armv8_cortex_a53/type:8
> /sys/devices/armv8_cortex_a72/type:9
> /sys/devices/armv8_cortex_a53/cpus:0-3
> /sys/devices/armv8_cortex_a72/cpus:4-5
> 
> See, non-Apple HW. And now for a system with homogeneous CPUs:
> 
> $ grep . /sys/devices/armv8*/{type,cpus}
> /sys/devices/armv8_pmuv3_0/type:8
> /sys/devices/armv8_pmuv3_0/cpus:0-159
> 
> Still no type 4. I could go on for hours, I have plenty of HW around
> me!
> 
> So whatever your source of information is, it doesn't match reality.
> Our PMUs are numbered arbitrarily, and have been so for... a very long
> time. At least since perf_pmu_register has supported dynamic
> registration (see 2e80a82a49c4c).
> 
> Thanks,
> 
> 	M.
> 


I agree with Marc,
on s390 we have 5 different PMUs and all have arbitrary numbers
and have totally different features:

# ll /sys/devices/{cpum,pai}*/type
-r--r--r-- 1 root root 4096 May 30 09:33 /sys/devices/cpum_cf_diag/type
-r--r--r-- 1 root root 4096 May 30 09:33 /sys/devices/cpum_cf/type
-r--r--r-- 1 root root 4096 May 30 09:33 /sys/devices/cpum_sf/type
-r--r--r-- 1 root root 4096 May 30 09:33 /sys/devices/pai_crypto/type
-r--r--r-- 1 root root 4096 May 30 09:33 /sys/devices/pai_ext/type
# grep . /sys/devices/{cpum,pai}*/type
/sys/devices/cpum_cf_diag/type:9
/sys/devices/cpum_cf/type:8
/sys/devices/cpum_sf/type:4
/sys/devices/pai_crypto/type:10
/sys/devices/pai_ext/type:11
# 

Thanks Thomas
-- 
Thomas Richter, Dept 3303, IBM s390 Linux Development, Boeblingen, Germany
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