Re: [PATCH v2 2/3] perf x86: Add compaction function for uncore attributes

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Dec 10, 2019 at 12:14:50PM +0300, roman.sudarikov@linux.intel.com wrote:
> From: Roman Sudarikov <roman.sudarikov@linux.intel.com>
> 
> In current design, there is an implicit assumption that array of pointers
> to uncore type attributes is NULL terminated. However, not all attributes
> are mandatory for each Uncore unit type, e.g. "events" is required for
> IMC but doesn't exist for CHA. That approach correctly supports only one
> optional attribute which also must be the last in the row.
> The patch removes limitation by safely removing embedded NULL elements.
> 
> Co-developed-by: Alexander Antonov <alexander.antonov@intel.com>
> Signed-off-by: Alexander Antonov <alexander.antonov@intel.com>
> Signed-off-by: Roman Sudarikov <roman.sudarikov@linux.intel.com>
> ---
>  arch/x86/events/intel/uncore.c | 22 ++++++++++++++++++++++
>  1 file changed, 22 insertions(+)
> 
> diff --git a/arch/x86/events/intel/uncore.c b/arch/x86/events/intel/uncore.c
> index 24e120289018..a05352c4fc01 100644
> --- a/arch/x86/events/intel/uncore.c
> +++ b/arch/x86/events/intel/uncore.c
> @@ -923,6 +923,22 @@ static void uncore_types_exit(struct intel_uncore_type **types)
>  		uncore_type_exit(*types);
>  }
>  
> +static void uncore_type_attrs_compaction(struct intel_uncore_type *type)
> +{
> +	int i, j;
> +	int size = ARRAY_SIZE(type->attr_groups);
> +
> +	for (i = 0, j = 0; i < size; i++) {
> +		if (!type->attr_groups[i])
> +			continue;
> +		if (i > j) {
> +			type->attr_groups[j] = type->attr_groups[i];
> +			type->attr_groups[i] = NULL;
> +		}
> +		j++;
> +	}
> +}

GregKH had objections to us playing silly games like that and made us
use is_visible() for the regular PMU driver. Also see commit:

  baa0c83363c7 ("perf/x86: Use the new pmu::update_attrs attribute group")