Re: [RFC PATCH] perf tools: Don't set inherit bit for system wide evsel

[Adrian Hunter <adrian.hunter@intel.com>]

On 26/10/15 11:08, Wangnan (F) wrote:
> 
> 
> On 2015/10/24 0:17, Arnaldo Carvalho de Melo wrote:
>> Em Fri, Oct 23, 2015 at 09:58:20PM +0800, pi3orama escreveu:
>>>
>>> 发自我的 iPhone
>>>
>>>> 在 2015年10月23日，下午9:51，Arnaldo Carvalho de Melo <acme@kernel.org>
>>>> 写道：
>>>>
>>>> Em Fri, Oct 23, 2015 at 10:43:49AM +0000, Wang Nan escreveu:
>>>>> Inherit bit is useless for a system wide evsel [1]. Further kernel
>>>>> improvements are giving more constrain [2] on inherit events. This
>>>>> patch set inherit bit to 0 to avoid potential constrains.
>>>>>
>>>>> [1]
>>>>> http://lkml.kernel.org/r/20151022124142.GQ17308@twins.programming.kicks-ass.net
>>>>>
>>>>> [2]
>>>>> http://lkml.kernel.org/r/1445559014-4667-1-git-send-email-ast@kernel.org
>>>>>
>>>>> Signed-off-by: Wang Nan <wangnan0@huawei.com>
>>>>> Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
>>>>> Cc: Alexei Starovoitov <ast@plumgrid.com>
>>>>> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
>>>>> Cc: Li Zefan <lizefan@huawei.com>
>>>>> Cc: pi3orama@163.com
>>>>> Link:
>>>>> http://lkml.kernel.org/n/ebpf-0tgilipxoo6fiebcxu3ft866@git.kernel.org
>>>>> ---
>>>>>
>>>>> evsel->system_wide doesn't correct reflect whether this evsel is system
>>>>> wide or not, so checks pid when invoking perf_event_open, and it is
>>>>> always correct.
>>>> Can't we do this at perf_evlist__config() or perf_evsel__config() time?
>>> perf_evlist_config() is excluded because perf record doesn't use it.
>> Yeah, we need to make it use it :-\
> 
> Its my fault that perf record *does* use perf_evlist__config(), but 'perf stat'
> doesn't.
> 
>>  
>>>> We have record_opts at perf_evsel__config() time and I think we should
>>>> leave changing the attr at perf_evsel__open() time for feature
>>>> fallbacks, i.e. something we will only know when trying to use, which is
>>>> different from this inherit-on-syswide case, that we know far in advance
>>>> we will not need.
>>> I tried to set this bit based on evsel->system_wide but it seems not
>>> reliable
>>> as it should be, so I was wondering whether it is designed for other use.
>>> I will look
>>> into this next week.
> 
> evsel->system_wide is introduced by commit
> bf8e8f4b832972c76d64ab2e2837a48397144887
> (perf evlist: Add 'system_wide' option), but Adrian only introduced a new field
> into perf, doesn't really make it active. Until now the only user of it is
> arch/x86/util/intel-pt.c, but I'm not very sure the reason for IPT to use that
> field.
> 
> If I understand correctly, it should be okay for a normal system wide evsel
> to have
> this var set. I'll try another RFC for it.

evsel->system_wide is for mixing evsels that aren't system-wide with ones
that are.

It might work to set it for all system-wide evsels but you will have to
check the code and test it, because that would be using it in a new way
that has never been tested.