Subject: Re: [PATCH v5 5/7] powerpc/powernv: add event attribute and group to nest pmu

From: Madhavan Srinivasan <[email-masked]>
Date: Thu, 23 Jul 2015 12:02:05 +0530

On Wednesday 22 July 2015 10:14 AM, Daniel Axtens wrote:
> On Thu, 2015-07-16 at 16:43 +0530, Madhavan Srinivasan wrote:
>> Add code to create event/format attributes and attribute groups for
>> each nest pmu.
>>
>> Cc: Michael Ellerman <[email-masked]>
>> Cc: Benjamin Herrenschmidt <[email-masked]>
>> Cc: Paul Mackerras <[email-masked]>
>> Cc: Anton Blanchard <[email-masked]>
>> Cc: Sukadev Bhattiprolu <[email-masked]>
>> Cc: Anshuman Khandual <[email-masked]>
>> Cc: Stephane Eranian <[email-masked]>
>> Signed-off-by: Madhavan Srinivasan <[email-masked]>
>> ---
>>  arch/powerpc/perf/nest-pmu.c | 65 ++++++++++++++++++++++++++++++++++++++++++++
>>  1 file changed, 65 insertions(+)
>>
>> diff --git a/arch/powerpc/perf/nest-pmu.c b/arch/powerpc/perf/nest-pmu.c
>> index c4c08e4dee55..f3418bdec1cd 100644
>> --- a/arch/powerpc/perf/nest-pmu.c
>> +++ b/arch/powerpc/perf/nest-pmu.c
>> @@ -13,6 +13,17 @@
>>  static struct perchip_nest_info p8_nest_perchip_info[P8_NEST_MAX_CHIPS];
>>  static struct nest_pmu *per_nest_pmu_arr[P8_NEST_MAX_PMUS];
>>  
>> +PMU_FORMAT_ATTR(event, "config:0-20");
>> +static struct attribute *p8_nest_format_attrs[] = {
>> +	&format_attr_event.attr,
>> +	NULL,
>> +};
>> +
>> +static struct attribute_group p8_nest_format_group = {
>> +	.name = "format",
>> +	.attrs = p8_nest_format_attrs,
>> +};
>> +
>>  static int nest_event_info(struct property *pp, char *name,
>>  			struct nest_ima_events *p8_events, int string, u32 val)
>>  {
>> @@ -46,6 +57,56 @@ static int nest_event_info(struct property *pp, char *name,
>>  	return 0;
>>  }
>>  
>> +/*
>> + * Populate event name and string in attribute
>> + */
>> +static struct attribute *dev_str_attr(const char *name, const char *str)
>> +{
>> +	struct perf_pmu_events_attr *attr;
>> +
>> +	attr = kzalloc(sizeof(*attr), GFP_KERNEL);
>> +
>> +	sysfs_attr_init(&attr->attr.attr);
>> +
>> +	attr->event_str = str;
>> +	attr->attr.attr.name = name;
>> +	attr->attr.attr.mode = 0444;
>> +	attr->attr.show = perf_event_sysfs_show;
>> +
>> +	return &attr->attr.attr;
> So I asked you about this before, and you pointed me to
> perf_event_sysfs_show. Looking at that in kernel/events/core.c, it looks
> like that uses container_of to pull out the perf_pmu_events_attr. So I
> guess that is at least mostly correct.
>
> I'm hoping something else uses container_of to pull out attr->attr, so
> that they can actually grab the attr->attr.show function pointer, so
> that perf_event_sysfs_show actually gets called. Where would that be?

OK, what we return is the device attribute struct which also have sysfs_ops.
So ->show and ->store are those entries in the strucutre and here we only
populate show ops using perf_event_sysfs_show. Now at the time of
pmu registering, we end up calling device_add->device_create_file->
sysfs_create_file which will end up adding a sysfs device file linked to
this
->show ops.

>> +}
>> +
>> +static int update_events_in_group(
>> +	struct nest_ima_events *p8_events, int idx, struct nest_pmu *pmu)
>> +{
>> +	struct attribute_group *attr_group;
>> +	struct attribute **attrs;
>> +	int i;
>> +
>> +	/*
>> +	 * Allocate memory for both event attribute group and for
>> +	 * event attributes array.
>> +	 */
>> +	attr_group = kzalloc(((sizeof(struct attribute *) * (idx + 1)) +
>> +				sizeof(*attr_group)), GFP_KERNEL);
>> +	if (!attr_group)
>> +		return -ENOMEM;
>> +
>> +	/*
>> +	 * Assign memory for event attribute array
>> +	 */
>> +	attrs = (struct attribute **)(attr_group + 1);
>> +	attr_group->name = "events";
>> +	attr_group->attrs = attrs;
> I am super uncomfortable with this block, especially the assignment to
> attrs. I *think* you're trying to allocate an attribute group and a set
> of attributes, but you've combined the allocation into one big
> contiguous chunk, and then you're trying to tease them apart. Is that
> necessary? Could it be two allocs, one for the attribute_group and one
> for the attribute?

I wanted to avoid two function calls here, but this is not a hot path
This happens at the pmu init time (booting), so I guess we can have
two allocs here.