linux-kernel - Re: [V14 3/8] drivers: perf: arm_pmuv3: Enable branch stack sampling framework

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Message-ID: <c8ddb80b-4228-4ed3-bc54-fba334b70c2b@arm.com>
Date:   Tue, 21 Nov 2023 14:43:54 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     James Clark <james.clark@....com>
Cc:     Mark Brown <broonie@...nel.org>, Rob Herring <robh@...nel.org>,
        Marc Zyngier <maz@...nel.org>,
        Suzuki Poulose <suzuki.poulose@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        linux-perf-users@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        will@...nel.org, catalin.marinas@....com, mark.rutland@....com
Subject: Re: [V14 3/8] drivers: perf: arm_pmuv3: Enable branch stack sampling
 framework



On 11/15/23 15:07, James Clark wrote:
> 
> 
> On 15/11/2023 05:44, Anshuman Khandual wrote:
>> On 11/14/23 15:28, James Clark wrote:
>>>
>>>
>>> On 14/11/2023 05:13, Anshuman Khandual wrote:
>>>> Branch stack sampling support i.e capturing branch records during execution
>>>> in core perf, rides along with normal HW events being scheduled on the PMU.
>>>> This prepares ARMV8 PMU framework for branch stack support on relevant PMUs
>>>> with required HW implementation.
>>>>
>>>
>>> [...]
>>>
>>>> - All armv8pmu_branch_xxxx() stub definitions have been moved inside
>>>>   include/linux/perf/arm_pmuv3.h for easy access from both arm32 and
>>>>   arm64 platforms
>>>>
>>>
>>> This causes lots of W=1 build errors because the prototypes are in
>>> arm_pmuv3.h, but arm_brbe.c doesn't include it.
>>
>> I guess these are the W=1 warnings you mentioned above.
>>
>> drivers/perf/arm_brbe.c:11:6: warning: no previous prototype for ‘armv8pmu_branch_reset’ [-Wmissing-prototypes]
>>    11 | void armv8pmu_branch_reset(void)                                                                                                                                                                   
>>       |      ^~~~~~~~~~~~~~~~~~~~~                                                                                                                                                                         
>> drivers/perf/arm_brbe.c:190:6: warning: no previous prototype for ‘armv8pmu_branch_save’ [-Wmissing-prototypes]      
>>   190 | void armv8pmu_branch_save(struct arm_pmu *arm_pmu, void *ctx)                      
>>       |      ^~~~~~~~~~~~~~~~~~~~                                                                                                                                                                          
>> drivers/perf/arm_brbe.c:236:6: warning: no previous prototype for ‘armv8pmu_branch_attr_valid’ [-Wmissing-prototypes]
>>   236 | bool armv8pmu_branch_attr_valid(struct perf_event *event)                                                                                                                                          
>>       |      ^~~~~~~~~~~~~~~~~~~~~~~~~~                                                                                                                                                                    
>> drivers/perf/arm_brbe.c:269:5: warning: no previous prototype for ‘armv8pmu_task_ctx_cache_alloc’ [-Wmissing-prototypes]
>>   269 | int armv8pmu_task_ctx_cache_alloc(struct arm_pmu *arm_pmu)                            
>>       |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~                                                            
>> drivers/perf/arm_brbe.c:279:6: warning: no previous prototype for ‘armv8pmu_task_ctx_cache_free’ [-Wmissing-prototypes]
>>   279 | void armv8pmu_task_ctx_cache_free(struct arm_pmu *arm_pmu)                       
>>       |      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~                                                            
>> drivers/perf/arm_brbe.c:303:6: warning: no previous prototype for ‘armv8pmu_branch_probe’ [-Wmissing-prototypes]
>>   303 | void armv8pmu_branch_probe(struct arm_pmu *armpmu)                                
>>       |      ^~~~~~~~~~~~~~~~~~~~~                                                                   
>> drivers/perf/arm_brbe.c:449:6: warning: no previous prototype for ‘armv8pmu_branch_enable’ [-Wmissing-prototypes]
>>   449 | void armv8pmu_branch_enable(struct arm_pmu *arm_pmu)                             
>>       |      ^~~~~~~~~~~~~~~~~~~~~~                                                                  
>> drivers/perf/arm_brbe.c:474:6: warning: no previous prototype for ‘armv8pmu_branch_disable’ [-Wmissing-prototypes]
>>   474 | void armv8pmu_branch_disable(void)                                                           
>>       |      ^~~~~~~~~~~~~~~~~~~~~~~                                                                 
>> drivers/perf/arm_brbe.c:717:6: warning: no previous prototype for ‘armv8pmu_branch_read’ [-Wmissing-prototypes]
>>   717 | void armv8pmu_branch_read(struct pmu_hw_events *cpuc, struct perf_event *event)
>>
>> Branch helpers are used in ARM PMU V3 driver i.e drivers/perf/arm_pmuv3.c.
>> Whether the actual BRBE helper definitions, or their fallback stubs (when
>> CONFIG_ARM64_BRBE is not enabled), need to be accessible from arm_pmuv3.c
>> driver not from brbe.c implementations itself.
>>
>>>
>>> It seems like the main reason you can't include arm_brbe.h in arm32 code
>>> is because there are a load of inline functions and references to
>>> registers in there. But these are only used in arm_brbe.c, so they don't
>>
>> Right, arm32 should not be exposed to BRBE internals via arm_brbe.h header.
>>
>>> need to be in the header anyway.
>>
>> Right, these are only used in arm_brbe.c
>>
>>>
>>> If you removed the code from the header and moved it to the source file
>>> you could move the brbe prototypes to the brbe header and it would be a
>>> bit cleaner and more idiomatic.
>>
>> Alight, how about the following changes - build tested on arm32 and arm64.
>>
>> - Move BRBE helpers from arm_brbe.h into arm_brbe.c
>> - Move armv8_pmu_xxx() declaration inside arm_brbe.h for arm64 (CONFIG_ARM64_BRBE)
>> - Move armv8_pmu_xxx() stub definitions inside arm_pmuv3.c for arm32 (!CONFIG_ARM64_BRBE)
>> - Include arm_brbe.h header both in arm_pmuv3.c and arm_brbe.c
> 
> Agree to them all except:
> 
>   - Move armv8_pmu_xxx() stub definitions inside arm_pmuv3.c for arm32
> (!CONFIG_ARM64_BRBE)
> 
> Normally you put the stubs right next to the prototypes with #else, so
> in this case both would be in arm_brbe.h. Not sure what the reason for
> splitting them here is? You already said "include arm_brbe.h in
> arm_pmuv3.c", so that covers arm32 too.

Not any particular strong reason for the split as such, will move these
stubs to the header as well. BRBE header includes <linux/perf/arm_pmu.h>
which causes the following redefinition warning for the pr_fmt().

drivers/perf/arm_brbe.c:11: warning: "pr_fmt" redefined
   11 | #define pr_fmt(fmt) "brbe: " fmt
      | 
In file included from ./include/linux/kernel.h:31,
                 from ./include/linux/interrupt.h:6,
                 from ./include/linux/perf/arm_pmu.h:11,
                 from drivers/perf/arm_brbe.h:10,
                 from drivers/perf/arm_brbe.c:9:
./include/linux/printk.h:345: note: this is the location of the previous definition
  345 | #define pr_fmt(fmt) fmt

Although it should be okay to just drop this custom pr_fmt() from BRBE.