Messages in this thread | | | From | Namhyung Kim <> | Date | Wed, 23 Mar 2022 13:54:23 -0700 | Subject | Re: [PATCH] perf/stat: Fix perf stat for forked applications |
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On Wed, Mar 23, 2022 at 4:07 AM Jiri Olsa <olsajiri@gmail.com> wrote: > > On Thu, Mar 17, 2022 at 04:53:46PM +0100, Thomas Richter wrote: > > I have run into the following issue: > > > > # perf stat -a -e new_pmu/INSTRUCTION_7/ -- mytest -c1 7 > > > > Performance counter stats for 'system wide': > > > > 0 new_pmu/INSTRUCTION_7/ > > > > 0.000366428 seconds time elapsed > > # > > > > The new PMU for s390 counts the execution of certain CPU instructions. > > The root cause is the extremely small run time of the > > mytest program. It just executes some assembly instructions > > and then exits. In above invocation the instruction is executed > > exactly one time (-c1 option). The PMU is expected to report this one > > time execution by a counter value of one, but fails to do so > > in some cases, not all. > > > > Debugging reveals the invocation of the child process is done > > *before* the counter events are installed and enabled. Tracing > > reveals that sometimes the child process starts and exits before > > the event is installed on all CPUs. The more CPUs the machine has, > > the more often this miscount happens. > > > > Fix this by reversing the start of the work load after the events > > have been installed on the specified CPUs. Now the comment also > > matches the code. > > > > Output after: > > # perf stat -a -e new_pmu/INSTRUCTION_7/ -- mytest -c1 7 > > > > Performance counter stats for 'system wide': > > > > 1 new_pmu/INSTRUCTION_7/ > > > > 0.000366428 seconds time elapsed > > # > > > > Now the correct result is reported rock solid all the time regardless > > how many CPUs are online. > > > > Fixes: acf2892270dc ("perf stat: Use perf_evlist__prepare/start_workload()) > > > > Signed-off-by: Thomas Richter <tmricht@linux.ibm.com> > > Acked-by: Sumanth Korikkar <sumanthk@linux.ibm.com> > > Cc: Namhyung Kim <namhyung@kernel.org> > > --- > > tools/perf/builtin-stat.c | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/tools/perf/builtin-stat.c b/tools/perf/builtin-stat.c > > index 3f98689dd687..60baa3dadc4b 100644 > > --- a/tools/perf/builtin-stat.c > > +++ b/tools/perf/builtin-stat.c > > @@ -955,10 +955,10 @@ static int __run_perf_stat(int argc, const char **argv, int run_idx) > > * Enable counters and exec the command: > > */ > > if (forks) { > > - evlist__start_workload(evsel_list); > > err = enable_counters(); > > if (err) > > return -1; > > + evlist__start_workload(evsel_list); > > right, without -a the event has enable_on_exec so the race does not > matter, but it's a problem for system wide with fork
Agreed. Also we may move the enable_counters() and the clock code out of the if block to be shared with the else block.
Acked-by: Namhyung Kim <namhyung@kernel.org>
Thanks, Namhyung
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