linux-kernel - Re: [PATCH] perf parse events: Fix invalid precise

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Date:   Wed, 15 Nov 2017 09:00:03 +0800
From:   zhangmengting <zhangmengting@...wei.com>
To:     Jiri Olsa <jolsa@...hat.com>
CC:     <namhyung@...nel.org>, <alexander.shishkin@...ux.intel.com>,
        <acme@...nel.org>, <Linux-kernel@...r.kernel.org>,
        <linux-perf-users@...r.kernel.org>, <huawei.libin@...wei.com>,
        <wangnan0@...wei.com>
Subject: Re: [PATCH] perf parse events: Fix invalid precise_ip handling

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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