Re: [PATCH] perf parse events: Fix invalid precise_ip handling

[zhangmengting]

On 2017/11/20 15:33, Jiri Olsa wrote:
> On Wed, Nov 15, 2017 at 09:00:03AM +0800, zhangmengting wrote:
> > Hi Jiri, thanks for your detailed review, please see my comments inline.
> >
> >
> > On 2017/11/10 18:39, Jiri Olsa wrote:
> > > On Fri, Nov 10, 2017 at 04:28:37PM +0800, Mengting Zhang wrote:
> > >
> > > SNIP
> > >
> > > > diff --git a/tools/perf/util/parse-events.c b/tools/perf/util/parse-events.c
> > > > index 39b1596..25225f4 100644
> > > > --- a/tools/perf/util/parse-events.c
> > > > +++ b/tools/perf/util/parse-events.c
> > > > @@ -1369,6 +1369,32 @@ struct event_modifier {
> > > >    	int pinned;
> > > >    };
> > > > +static int perf_get_max_precise_ip(void)
> > > > +{
> > > > +       int max_precise_ip = 0;
> > > > +       struct perf_event_attr attr = {
> > > > +               .type   = PERF_TYPE_HARDWARE,
> > > > +               .config = PERF_COUNT_HW_CPU_CYCLES,
> > > > +       };
> > > > +
> > > > +       event_attr_init(&attr);
> > > > +
> > > > +       attr.precise_ip = 3;
> > > > +       attr.sample_period = 1;
> > > > +
> > > > +       while (attr.precise_ip != 0) {
> > > > +               int fd = sys_perf_event_open(&attr, 0, -1, -1, 0);
> > > > +               if (fd != -1){
> > > > +                       close(fd);
> > > > +                       break;
> > > > +               }
> > > > +               --attr.precise_ip;
> > > > +       }
> > > > +       max_precise_ip = attr.precise_ip;
> > > > +
> > > > +       return max_precise_ip;
> > > > +}
> > > we already have a function for that, please check perf_event_attr__set_max_precise_ip
> > Yeah, I've checked that function. But perf_event_attr__set_max_precise_ip()
> > will change attr.precise_ip
> > into the max precise ip available.
> >
> > In this case, perf should only check whether the user-specified precise_ip
> > is greater than the max
> > precise_ip without changing it into maximum.  Here, introduce
> > perf_get_max_precise_ip() to return
> > the max precise ip and do not change attr.precise_ip.
> >
> > But you reminds me that perf_get_max_precise_ip() can be simplied.
> well both do the same.. probe kernel for max precise level,
> so we can keep just one function for that

OKay, I will just keep that function for probing max precise level.

> > > also I think the precise level is not generic for all the events,
> > > so you should check it for specific perf_event_attr later, when
> > > the attr is ready, not in modifier parsing
> > You are right, and I would check it for specific perf_event_attr.
> >
> > BTW, I have a question. If the user-specified precise_ip is greater than the
> > max precise_ip, I wonder
> > whether it is better to adjust the user-specified precise_ip to the maximum
> > available.
> no, I think that user defined precise level should stay the
> way the user wants it.. we don't want more angry users ;-)

Humm, I am sorry for being unclear.
If the user defined precise level is greater than the max precise level,
I think there are two ways to deal with it.
1.  return EINVAL to indicate the invalid precise_ip setting;
2.  adjust to the max precise level available and give message to 
indicate the adjustment.

Since we should check user-defined precise level in perf_evsel__config(),
when the attr is ready, I think there is a problem with method 1, if we 
keep the
user defined precise level stay the way the user wants it.

With method 1, we have to let perf_evsel__config() return value and show 
errno.
And this change will affect many related functions, such as 
perf_evlist__config(), and files.

With method 2, we don't need to change the return type of 
perf_evsel__config().

Am I right?
> jirka
>
> .