Re: [PATCH V5 2/7] arm64/perf: Update struct arm_pmu for BRBE

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 11/18/22 23:17, Mark Rutland wrote:
> 
> Hi Anshuman,
> 
> Apologies for the delayi n reviewing this.
> 
> On Fri, Nov 18, 2022 at 12:09:07PM +0530, Anshuman Khandual wrote:
>> On 11/9/22 17:00, Suzuki K Poulose wrote:
>>> On 07/11/2022 06:25, Anshuman Khandual wrote:
>>>> Although BRBE is an armv8 speciifc HW feature, abstracting out its various
>>>> function callbacks at the struct arm_pmu level is preferred, as it cleaner
>>>> , easier to follow and maintain.
>>>>
>>>> Besides some helpers i.e brbe_supported(), brbe_probe() and brbe_reset()
>>>> might not fit seamlessly, when tried to be embedded via existing arm_pmu
>>>> helpers in the armv8 implementation.
>>>>
>>>> Updates the struct arm_pmu to include all required helpers that will drive
>>>> BRBE functionality for a given PMU implementation. These are the following.
>>>>
>>>> - brbe_filter    : Convert perf event filters into BRBE HW filters
>>>> - brbe_probe    : Probe BRBE HW and capture its attributes
>>>> - brbe_enable    : Enable BRBE HW with a given config
>>>> - brbe_disable    : Disable BRBE HW
>>>> - brbe_read    : Read BRBE buffer for captured branch records
>>>> - brbe_reset    : Reset BRBE buffer
>>>> - brbe_supported: Whether BRBE is supported or not
>>>>
>>>> A BRBE driver implementation needs to provide these functionalities.
>>>
>>> Could these not be hidden from the generic arm_pmu and kept in the
>>> arm64 pmu backend  ? It looks like they are quite easy to simply
>>> move these to the corresponding hooks in arm64 pmu.
>>
>> We have had this discussion multiple times in the past [1], but I still
>> believe, keeping BRBE implementation hooks at the PMU level rather than
>> embedding them with other PMU events handling, is a much better logical
>> abstraction.
>>
>> [1] https://lore.kernel.org/all/c3804290-bdb1-d1eb-3526-9b0ce4c8e8b1@arm.com/
>>
>> --------------------------------------------------------------------------
>>>
>>> One thing to answer in the commit msg is why we need the hooks here.  
>>> Have we concluded that adding BRBE hooks to struct arm_pmu for what is 
>>> an armv8 specific feature is the right approach? I don't recall 
>>> reaching that conclusion.
>>
>> Although it might be possible to have this implementation embedded in
>> the existing armv8 PMU implementation, I still believe that the BRBE
>> functionalities abstracted out at the arm_pmu level with a separate
>> config option is cleaner, easier to follow and to maintain as well.
>>
>> Besides some helpers i.e brbe_supported(), brbe_probe() and brbe_reset()
>> might not fit seamlessly, when tried to be embedded via existing arm_pmu
>> helpers in the armv8 implementation.
>>
>> Nonetheless if arm_pmu based additional BRBE helpers is absolutely a no
>> go for folks here in general, will explore arm64 based implementation.
>> ----------------------------------------------------------------------------
>>
>> I am still waiting for maintainer's take on this issue. I will be happy to
>> rework this series to move all these implementation inside arm64 callbacks
>> instead, if that is required or preferred by the maintainers. But according
>> to me, this current abstraction layout is much better.
> 
> To be honest, I'm not sure what's best right now; but at the moment it's not
> clear to me why this couldn't fit within the existing hooks.
> 
> Above you say brbe_supported() / brbe_probe() / brbe_reset() didn't fit
> seamlessly; can you give an example of problem? I think I'm missing something
> obvious.

I tried to move them inside armv8 implementation callbacks.

arm64_pmu_brbe_supported() can be moved inside __armv8_pmuv3_map_event(), so that
event viability can be validated during armpmu_event_init(). arm64_pmu_brbe_probe()
can be moved inside __armv8pmu_probe_pmu() as you have suggested earlier on another
thread. arm64_pmu_brbe_reset() can also be moved inside armv8pmu_enable_event(),
and also armv8pmu_reset().

The only problem being armpmu_sched_task() where earlier we had BRBE reset, but I
guess it can be replaced with entire PMU reset which does the BRBE reset as well ?

static void armpmu_sched_task(struct perf_event_context *ctx, bool sched_in)
{
        struct arm_pmu *armpmu = to_arm_pmu(ctx->pmu);

        if (sched_in)
                armpmu->reset(armpmu);
}