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

[Jiri Olsa]

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@xxxxxxxxxxxxxxx>
> ---
>  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


> +		fd = FD(leader, 0, thread);
> +

so how do we group following events?

  cstate_pkg/c2-residency/ - cpumask 0
  msr/tsc/                 - all cpus

cpu 0 is fine.. the rest I have no idea ;-)

that's why metrics use the :W, that disables grouping on failure

jirka

>  	BUG_ON(fd == -1);
>  
>  	return fd;
> -- 
> 2.17.1
>