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

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Message-ID: <1a249a21-5e93-7434-8b6f-08241513dceb@huawei.com>
Date:   Tue, 21 Nov 2017 16:30:09 +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

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
>
> .
>