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

[Wangnan (F)]

On 2015/10/24 0:17, Arnaldo Carvalho de Melo wrote:
> Em Fri, Oct 23, 2015 at 09:58:20PM +0800, pi3orama escreveu:
> > åèæç iPhone
> >
> > > å 2015å10æ23æïäå9:51ïArnaldo Carvalho de Melo <acme@xxxxxxxxxx> åéï
> > >
> > > 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@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> > > > [2] http://lkml.kernel.org/r/1445559014-4667-1-git-send-email-ast@xxxxxxxxxx
> > > >
> > > > Signed-off-by: Wang Nan <wangnan0@xxxxxxxxxx>
> > > > Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> > > > Cc: Alexei Starovoitov <ast@xxxxxxxxxxxx>
> > > > Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
> > > > Cc: Li Zefan <lizefan@xxxxxxxxxx>
> > > > Cc: pi3orama@xxxxxxx
> > > > Link: http://lkml.kernel.org/n/ebpf-0tgilipxoo6fiebcxu3ft866@xxxxxxxxxxxxxx
> > > > ---
> > > >
> > > > 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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