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

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

On 05/19/2010 07:08 PM, Lin Ming wrote:
> On Thu, 2010-05-20 at 09:33 +0800, Corey Ashford wrote:
>> 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) ?
>
> Place the id code in attr->config.
>
>>
>> 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.
>>
>
> Each event is a directory in the sysfs, so we can put all the event
> "attributes" under it.
>
> For your example,
> /sys/devices/system/node/events/<event-name>/id
> /sys/devices/system/node/events/<event-name>/memory_range
> ....
>
> Then we can read these attributes and pass the value into the syscall.
>

I'm not sure I made the example clear.

Let's say I have a memory controller event called memory_write and it 
has two attributes: low_addr and high_addr... writes to addresses 
between the low_addr and high_addr will increment the counter.

As a user, I want to be able to specify a particular memory range, let's 
say 0x1000000..0x2000000

A sysfs structure like this might be constructed:
/sys/devices/system/node/events/memory_write
/sys/devices/system/node/events/memory_write/attr
/sys/devices/system/node/events/memory_write/attr/low_addr
/sys/devices/system/node/events/memory_write/attr/low_addr/min
/sys/devices/system/node/events/memory_write/attr/low_addr/max

In another posting I had also added a bit shift value, but there are so 
few bits left in the attr->config, that I'm not sure this is a very 
extensible mechanism, but just for the sake of illustration of the basic 
idea, I'll add it here:
/sys/devices/system/node/events/memory_write/attr/low_addr/shift

Then the same thing is repeated for the other attribute:
/sys/devices/system/node/events/memory_write/attr/high_addr
/sys/devices/system/node/events/memory_write/attr/high_addr/min
/sys/devices/system/node/events/memory_write/attr/high_addr/max
/sys/devices/system/node/events/memory_write/attr/high_addr/shift

In this scenario, a user tool (like perf) would be able to see that 
there are attributes associated with the memory_write event, and it 
knows the names and range of allowed values for these attributes.  The 
shift value tells the tool how much to shift the attribute value before 
OR'ing it into the attr->config value.

If we find that more than 64 bits are needed for the event code plus the 
attribute values, perhaps shift values greater than 64 would denote to 
place the attribute bits into a new attr field, like 
attr->config_extra[shift / 64]

On the perf command line, then, a user could specify something like:

perf stat -e node::memory_write:low_addr=0x1000000:high_addr=0x2000000

What do you think?

- Corey