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

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Date:	Mon, 26 Oct 2015 17:08:13 +0800
From:	"Wangnan (F)" <wangnan0@...wei.com>
To:	Arnaldo Carvalho de Melo <acme@...nel.org>,
	pi3orama <pi3orama@....com>,
	Adrian Hunter <adrian.hunter@...el.com>
CC:	<linux-kernel@...r.kernel.org>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Li Zefan <lizefan@...wei.com>
Subject: Re: [RFC PATCH] perf tools: Don't set inherit bit for system wide
 evsel



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@...nel.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@...wei.com>
>>>> Cc: Arnaldo Carvalho de Melo <acme@...hat.com>
>>>> Cc: Alexei Starovoitov <ast@...mgrid.com>
>>>> Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>
>>>> Cc: Li Zefan <lizefan@...wei.com>
>>>> Cc: pi3orama@....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.

Thank you.

> Ok, thanks in advance, lemme go back looking at eBPF :-)
>
> - Arnaldo


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