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

[Arnaldo Carvalho de Melo]

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?

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.

- Arnaldo
 
> ---
>  tools/perf/util/evsel.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
> index 5566b16..e2d6c9a 100644
> --- a/tools/perf/util/evsel.c
> +++ b/tools/perf/util/evsel.c
> @@ -1337,6 +1337,7 @@ retry_sample_id:
>  
>  		for (thread = 0; thread < nthreads; thread++) {
>  			int group_fd;
> +			struct perf_event_attr attr;
>  
>  			if (!evsel->cgrp && !evsel->system_wide)
>  				pid = thread_map__pid(threads, thread);
> @@ -1346,7 +1347,10 @@ retry_open:
>  			pr_debug2("sys_perf_event_open: pid %d  cpu %d  group_fd %d  flags %#lx\n",
>  				  pid, cpus->map[cpu], group_fd, flags);
>  
> -			FD(evsel, cpu, thread) = sys_perf_event_open(&evsel->attr,
> +			attr = evsel->attr;
> +			if (pid == -1)
> +				attr.inherit = 0;
> +			FD(evsel, cpu, thread) = sys_perf_event_open(&attr,
>  								     pid,
>  								     cpus->map[cpu],
>  								     group_fd, flags);
> -- 
> 1.8.3.4
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