Re: [PATCH] perf evsel: Get group fd from CPU0 for system wide event

["Jin, Yao" <yao.jin@linux.intel.com>]

Hi Jiri,

On 5/5/2020 8:03 AM, Jiri Olsa wrote:
> On Sat, May 02, 2020 at 10:33:59AM +0800, Jin, Yao wrote:
> 
> SNIP
> 
>>>> @@ -1461,6 +1461,9 @@ static int get_group_fd(struct evsel *evsel, int cpu, int thread)
>>>>    	BUG_ON(!leader->core.fd);
>>>>    	fd = FD(leader, cpu, thread);
>>>> +	if (fd == -1 && leader->core.system_wide)
>>>
>>> fd does not need to be -1 in here.. in my setup cstate_pkg/c2-residency/
>>> has cpumask 0, so other cpus never get open and are 0, and the whole thing
>>> ends up with:
>>>
>>> 	sys_perf_event_open: pid -1  cpu 1  group_fd 0  flags 0
>>> 	sys_perf_event_open failed, error -9
>>>
>>> I actualy thought we put -1 to fd array but couldn't find it.. perhaps we should od that
>>>
>>>
>>
>> I have tested on two platforms. On KBL desktop fd is 0 for this case, but on
>> oncascadelakex server, fd is -1, so the BUG_ON(fd == -1) is triggered.
>>
>>>> +		fd = FD(leader, 0, thread);
>>>> +
>>>
>>> so how do we group following events?
>>>
>>>     cstate_pkg/c2-residency/ - cpumask 0
>>>     msr/tsc/                 - all cpus
>>>
>>
>> Not sure if it's enough to only use cpumask 0 because
>> cstate_pkg/c2-residency/ should be per-socket.
>>
>>> cpu 0 is fine.. the rest I have no idea ;-)
>>>
>>
>> Perhaps we directly remove the BUG_ON(fd == -1) assertion?
> 
> I think we need to make clear how to deal with grouping over
> events that comes for different cpus
> 
> 	so how do we group following events?
> 	
> 	   cstate_pkg/c2-residency/ - cpumask 0
> 	   msr/tsc/                 - all cpus
> 
> 
> what's the reason/expected output of groups with above events?
> seems to make sense only if we limit msr/tsc/ to cpumask 0 as well
> 
> jirka
> 

On 2-socket machine (e.g cascadelakex), "cstate_pkg/c2-residency/" is per-socket 
event and the cpumask is 0 and 24.

root@lkp-csl-2sp5 /sys/devices/cstate_pkg# cat cpumask
0,24

We can't limit it to cpumask 0. It should be programmed on CPU0 and CPU24 (the 
first CPU on each socket).

The "msr/tsc" are per-cpu event, it should be programmed on all cpus. So I don't 
think we can limit msr/tsc to cpumask 0.

The issue is how we deal with get_group_fd().

static int get_group_fd(struct evsel *evsel, int cpu, int thread)
{
         struct evsel *leader = evsel->leader;
         int fd;

         if (evsel__is_group_leader(evsel))
                 return -1;

         /*
          * Leader must be already processed/open,
          * if not it's a bug.
          */
         BUG_ON(!leader->core.fd);

         fd = FD(leader, cpu, thread);
         BUG_ON(fd == -1);

         return fd;
}

When evsel is "msr/tsc/",

FD(leader, 0, 0) is 3 (3 is the fd of "cstate_pkg/c2-residency/" on CPU0)
FD(leader, 1, 0) is -1
BUG_ON asserted.

If we just return group_fd(-1) for "msr/tsc", it looks like it's not a problem, 
is it?

Thanks
Jin Yao