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

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

Hi Jiri,

On 5/1/2020 6:23 PM, Jiri Olsa wrote:
> On Thu, Apr 30, 2020 at 09:34:51AM +0800, Jin Yao wrote:
>> A metric may consist of system wide event and non system-wide event.
>> The event group leader may be the system wide event.
>>
>> For example, the metric "C2_Pkg_Residency" consists of
>> "cstate_pkg/c2-residency" and "msr/tsc". The former counts on the first
>> CPU of socket (tagged system-wide) and the latter is per CPU.
>>
>> But "C2_Pkg_Residency" hits assertion failure on cascadelakex.
>>
>>   # perf stat -M "C2_Pkg_Residency" -a -- sleep 1
>>   perf: util/evsel.c:1464: get_group_fd: Assertion `!(fd == -1)' failed.
>>   Aborted
>>
>> get_group_fd(evsel, cpu, thread)
>> {
>> 	leader = evsel->leader;
>> 	fd = FD(leader, cpu, thread);
>> 	BUG_ON(fd == -1);
>> }
>>
>> Considering this case, leader is "cstate_pkg/c2-residency", evsel is
>> "msr/tsc" and cpu is 1. Because "cstate_pkg/c2-residency" is a system-wide
>> event and it's processed on CPU0, so FD(leader, 1, thread) must return an
>> invalid fd, then BUG_ON() may be triggered.
>>
>> This patch gets group fd from CPU0 for system wide event if
>> FD(leader, cpu, thread) returns invalid fd.
>>
>> With this patch,
>>
>>   # perf stat -M "C2_Pkg_Residency" -a -- sleep 1
>>
>>   Performance counter stats for 'system wide':
>>
>>          1000850802      cstate_pkg/c2-residency/  #      0.5 C2_Pkg_Residency
>>        201446161592      msr/tsc/
>>
>>         1.010637051 seconds time elapsed
>>
>> Fixes: 6a4bb04caacc ("perf tools: Enable grouping logic for parsed events")
>> Signed-off-by: Jin Yao <yao.jin@linux.intel.com>
>> ---
>>   tools/perf/util/evsel.c | 3 +++
>>   1 file changed, 3 insertions(+)
>>
>> diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
>> index 6a571d322bb2..cd6470f63d6f 100644
>> --- a/tools/perf/util/evsel.c
>> +++ b/tools/perf/util/evsel.c
>> @@ -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?

Thanks
Jin Yao

> that's why metrics use the :W, that disables grouping on failure
> 
> jirka
> 
>>   	BUG_ON(fd == -1);
>>   
>>   	return fd;
>> -- 
>> 2.17.1
>>
>