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....@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


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