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

[Jin, Yao]

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@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

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