Re: [PATCH 05/10] perf, tools, stat: Basic support for TopDown in perf stat

Andi Kleen
2015-12-16T21:21:11Z

> > +/*
> > + * Check whether we can use a group for top down.
> > + * Without a group may get bad results due to multiplexing.
> > + */
>
> That is not because you have a counter used by the NMI that
> you cannot group. If HT is off you have plenty of counters to
> do this.

Such a heuristic wouldn't work on Atom where there are no more
counters in this case.

> > +static bool check_group(bool *warn)
> > +{
> > + int n;
> > +
> > + if (sysctl__read_int("kernel/nmi_watchdog", &n) < 0)
> > + return false;
> > + if (n > 0) {
> > + *warn = true;
> > + return false;
> > + }
> > + return true;
> > +}
> > +
>
> I do not like this part very much. You are pulling in x86 specific
> knowledge into
> builtin_stat.c. Why not move this into an x86 specific file?

Done.

> > err = parse_events(evsel_list, transaction_attrs, NULL);
> > @@ -1511,6 +1579,36 @@ static int add_default_attributes(void)
> > return 0;
> > }
> >
> > + if (topdown_run) {
> > + char *str = NULL;
> > + bool warn = false;
> > +
> > + filter_events(topdown_attrs, &str, check_group(&warn));
> > + if (topdown_attrs[0] && str) {
> > + if (warn)
> > + fprintf(stderr,
> > + "nmi_watchdog enabled with topdown. May give wrong results.\n"
> > + "Disable with echo 0 > /proc/sys/kernel/nmi_watchdog\n");
>
> This is x86 specific. Why not just try it out and in case of error
> suggest checking
> if pinned system-wide events exist (such as NMI watchdog on x86). that would
> be more generic.

That's really complicated, i would have to tear down all state and then
resubmit all the events. I think just checking the NMI watchdog is good
enough. I couldn't give a sensible error message for the generic case
anyways.

-Andi
--
ak@linux.intel.com -- Speaking for myself only.