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

[zhangmengting]

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.
> 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.

Thanks
Mengting Zhang

> thanks,
> jirka
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