linux-kernel - Re: [GIT PULL 0/8] perf/pt -> Intel PT/BTS

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Date:	Wed, 01 Jul 2015 15:47:03 +0300
From:	Adrian Hunter <adrian.hunter@...el.com>
To:	Ingo Molnar <mingo@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>
CC:	linux-kernel@...r.kernel.org, Jiri Olsa <jolsa@...hat.com>,
	David Ahern <dsahern@...il.com>,
	Namhyung Kim <namhyung@...nel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Stephane Eranian <eranian@...gle.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>
Subject: Re: [GIT PULL 0/8] perf/pt -> Intel PT/BTS

On 01/07/15 11:19, Adrian Hunter wrote:
> On 30/06/15 16:23, Adrian Hunter wrote:
>> On 30/06/15 13:56, Ingo Molnar wrote:
>>>
>>> * Adrian Hunter <adrian.hunter@...el.com> wrote:
>>>
>>>>> Yeah, so I did a 'newbie test':
>>>>>
>>>>> I pulled the tree and saw that it has a tools/perf/Documentation/intel-bts.txt 
>>>>> file and started reading it.
>>>>>
>>>>> Based on its text:
>>>>>
>>>>>   The Intel BTS kernel driver creates a new PMU for Intel BTS.  The perf record
>>>>>   option is:
>>>>>
>>>>>         -e intel_bts//
>>>>>
>>>>>   Currently Intel BTS is limited to per-thread tracing so the --per-thread option
>>>>>   is also needed.
>>>>>
>>>>> I tried the following command which failed:
>>>>>
>>>>>   triton:~/tip> perf record -e intel_bts// --per-thread sleep 1
>>>>>   invalid or unsupported event: 'intel_bts//'
>>>>>   Run 'perf list' for a list of valid events
>>>>>
>>>>>    usage: perf record [<options>] [<command>]
>>>>>       or: perf record [<options>] -- <command> [<options>]
>>>>>
>>>>>       -e, --event <event>   event selector. use 'perf list' to list available events
>>>>>
>>>>> That's a really ... unhelpful message. If I typoed something I want to know that. 
>>>>> If the kernel does not support something, I want to know about that too. Tooling 
>>>>> telling me: "maybe you typoed something, maybe it's not supported, I really don't 
>>>>> care" is not very productive.
>>>>
>>>> That is not entirely true. The message says "Run 'perf list' for a list of valid 
>>>> events" which will tell you if the event is valid. So you can tell the 
>>>> difference between a typo and unsupported event.
>>>
>>> Yeah, but my point is: why doesn't the tool do this disambiguation for me? Tools 
>>> are hard enough to use as-is already, no need to put artificial roadblocks in the 
>>> path of first time users.
>>
>> That applies to all events e.g.
>>
>> # perf record -e sched:sched_swotch sleep 1
>> invalid or unsupported event: 'sched:sched_swotch'                                                                                       
>> Run 'perf list' for a list of valid events                                                                                               
>>                                                                                                                                          
>>  usage: perf record [<options>] [<command>]                                                                                              
>>     or: perf record [<options>] -- <command> [<options>]                                                                                 
>>                                                                                                                                          
>>     -e, --event <event>   event selector. use 'perf list' to list available events    
>>
>> So it is a general problem.
>>
>>>
>>>>> So this was with a distro kernel, and in the hope that I'm missing some magic 
>>>>> new kernel feature, I tried it the latest -tip kernel, but it still gives me 
>>>>> the same failure.
>>>>>
>>>>> So the test newbie user got stuck after wasting some time.
>>>>>
>>>>> Me as a kernel developer could probably figure it out, but that's not the 
>>>>> point: if newbies cannot discover and use our new features then it's as if 
>>>>> they didn't exist, and I'm not pulling non-existent features! ;-)
>>>>>
>>>>> Could we please improve all this?
>>>>
>>>> 'perf list' shows the event wasn't supported, so I am not sure what more the 
>>>> "newbie" could expect.  Do you have any suggestions?
>>>
>>> So I think a first time user would expect a clear message from the computer: what 
>>> was wrong with what he wrote and what should he do to fix it.
>>>
>>> Btw., here's the 'perf list' output from a system running the latest -tip kernel:
>>>
>>>   vega:~> uname -a
>>>   Linux vega 4.1.0-02935-g390ad45394a3-dirty #567 SMP PREEMPT Mon Jun 29 11:44:48 CEST 2015 x86_64 x86_64 x86_64 GNU/Linux
>>>   vega:~> perf list | grep -i bts
>>>   vega:~> 
>>>
>>> so is there any kernel feature dependency? It's unclear. If yes, it should be 
>>> mentioned in the document, and in the tooling output as well. If not then we have 
>>> a bug somewhere.
>>
>> I am not aware of any dependencies, apart from perf events itself.
>>
>> Are you sure you compiled perf tools with the new patches ;-)
>> And it is an Intel CPU?
>>

I am going to need to know what hardware it is and cpu feature flags i.e.
/proc/cpuinfo

Also ls /sys/bus/event_source/devices


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