Re: [RFC][PATCH v2 11/11] perf top: demo of how to use the sysfs interface

[Corey Ashford <cjashfor@linux.vnet.ibm.com>]

On 5/19/2010 6:17 PM, Lin Ming wrote:
> On Thu, 2010-05-20 at 02:18 +0800, Corey Ashford wrote:
>>
>> On 5/18/2010 6:49 PM, Lin Ming wrote:
>>> Just a temporary patch to show how to use the pmu sysfs interface...
>>>
>>> Signed-off-by: Lin Ming <ming.m.lin@intel.com>
>>> ---
>>>  tools/perf/builtin-top.c |   13 +++++++++++++
>>>  1 files changed, 13 insertions(+), 0 deletions(-)
>>>
>>> diff --git a/tools/perf/builtin-top.c b/tools/perf/builtin-top.c
>>> index adc179d..eaa9405 100644
>>> --- a/tools/perf/builtin-top.c
>>> +++ b/tools/perf/builtin-top.c
>>> @@ -1206,6 +1206,7 @@ static void start_counter(int i, int counter)
>>>  	struct perf_event_attr *attr;
>>>  	int cpu;
>>>  	int thread_index;
>>> +	int sys_fd;
>>>
>>>  	cpu = profile_cpu;
>>>  	if (target_tid == -1 && profile_cpu == -1)
>>> @@ -1226,9 +1227,21 @@ static void start_counter(int i, int counter)
>>>
>>>  	for (thread_index = 0; thread_index < thread_num; thread_index++) {
>>>  try_again:
>>> +		/*
>>> +		 * This is just an ugly demo of how to use the sysfs interface.
>>> +		 * You can also parse the <event-name> and open sys file as,
>>> +		 * sys_fd = open("/sys/devices/system/cpu/events/<event-name>/event_source/id", O_RDONLY);
>>> +		 */
>>
>> In this above case, does this sys_fd also specify the event I am going to open, in addition to its event source?  I'd assume not since event_source is a symlink to /sys/devices/system/cpu/event_source (right?)
> 
> Right, this sys_fd only specifies the event source.
> 
>> How do I specify the exact event id via the sysfs interface?
> 
> /sys/devices/system/cpu/events/<event-name>/id
> 
> But in this patch series, the event id sysfs interface is not used yet.

So, I would open that id and then read the id code and place it in attr->config or maybe place
the fd into attr (somewhere) ?

We also need to take into account event "attributes" - other data that is needed to configure a specific event.  For example, think about a memory controller which has a PMU can count events in a particular memory range; we need to be able to supply the memory range somehow, and I don't think that can be accomplished by passing in the fd of a sysfs file that we've opened.

- Corey