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