[fedora-arm] Linux kernel arm oprofile driver compilation problem

[William Cohen]

On 02/23/2012 05:12 AM, Will Deacon wrote:
> Hi Will,
> 
> On Thu, Feb 23, 2012 at 04:08:23AM +0000, William Cohen wrote:
>>
>> I was looking to see why the arm kernel builds for fedora 17 were failing with:
>>
>> arch/arm/oprofile/../../../drivers/oprofile/oprofile_perf.c:28:28: error: variably modified 'perf_events' at file scope
>>
>> The error points to the following line in oprofile_perf.c:
>>  
>> static struct perf_event **perf_events[nr_cpumask_bits];
>>
>> The nr_cpumask_bits is defined in linux/include/linux/cpumask.h with
>> the following snippet of code:
>>
>> #if NR_CPUS == 1
>> #define nr_cpu_ids              1
>> #else
>> extern int nr_cpu_ids;
>> #endif
>>
>> #ifdef CONFIG_CPUMASK_OFFSTACK
>> /* Assuming NR_CPUS is huge, a runtime limit is more efficient.  Also,
>>  * not all bits may be allocated. */
>> #define nr_cpumask_bits nr_cpu_ids
>> #else
>> #define nr_cpumask_bits NR_CPUS
>> #endif
>>
>> The problem is that several arm machines have more than one processor
>> (CONFIG_NR_CPUS>1), making nr_cpumask_bits be a variable rather than a
>> compile time constant.  Seems like the kernel should dynamically allocate
>> space for perf_events[nr_cpumask_bits] because of the variable nature
>> of nr_cpu_mask_bits.
> 
> That or use NR_CPUS instead (which will be the same as nr_cpu_ids on ARM).
> 
> One thing I'm curious about is why you have CONFIG_CPUMASK_OFFSTACK selected
> for the Fedora kernel? That implies you have DEBUG_PER_CPU_MAPS selected
> which, according to Kconfig, `adds a fair amount of code to kernel memory
> and decreases performance'.
> 
> Cheers,
> 
> Will

When I was building kernels locally I didn't see this problem because of:

# CONFIG_DEBUG_PER_CPU_MAPS is not set

So adding that to the config-arm-generic of the kenrel source rpm, should be a work around for this problem.

-Will