linux-kernel - Re: [PATCH/RFC] perf tools: Handle old kernels when opening perf event

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Date:	Fri, 09 Mar 2012 11:38:02 +0900
From:	Namhyung Kim <namhyung.kim@....com>
To:	linux-kernel@...r.kernel.org
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Mackerras <paulus@...ba.org>, Ingo Molnar <mingo@...e.hu>,
	Namhyung Kim <namhyung@...il.com>,
	Pekka Enberg <penberg@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH/RFC] perf tools: Handle old kernels when opening perf
 event

Hi,

2012-03-08 11:50 PM, Arnaldo Carvalho de Melo wrote:
> Em Thu, Mar 08, 2012 at 04:28:51 PM +0900, Namhyung Kim escreveu:
>> Changing default value of perf_guest back to false caused problems on old
>> kernels and its fix bc76efe64533 ("perf tools: Handle kernels that don't
>> support attr.exclude_{guest,host}") worked well for perf record/top.
>>
>> But I've just realized that using specific events on perf stat makes same
>> kind of troubles too. It's because the parse_events calls event_attr_init
>> for all events so that it would have exclude_guest set.
>>
>> Instead of fixing perf stat, I thought that changing perf_evsel__open()
>> is more appropriate. Please take a look and give comments - especially
>> on ->time_not_needed handling in builtin-record.c (I guess the original
>> code had a bug) and checking ->sample_id_all_missing inside of
>> perf_evsel__config (I believe checking it before perf_evsel__open is
>> meaningless since it will always have the same value - so I dropped it).
>
> One problem here is to have all those pr_debug calls down in the
> perf_evlist class, they need to be better conveyed to the user, and that
> depends on the kind of interface being used (stdio, TUI, GTK).
>
> One approach tried till the GTK patch was proposed was to just check the
> value of perf_browser or some UI fops table to do it at that level.
>
> But Pekka argued that we should allow tool writers to have more freedom
> than that, i.e. handle things as flexibly as possible.
>

Agreed.


> The way to do that, that I discussed with David Ahern some time ago, was
> to have per class errno enums, and then a per class strerror (really an
> strerror_r) that would map the integer error number to an string.
>
> Doing that we would also avoid having to bloat the python binding (or
> libperf at some point) with the pr_debug, etc machinery.
>

I remember that discussion. Yes, it'll show more verbose and helpful messages 
when user got in trouble. But I don't have an idea how it helps in this 
particular case - we anyway want to show user some message and keep going on. 
Could you explain your idea in little more detail?


> For instance, with your patch applied, try running
> tools/perf/python/twatch.py :-)
>

Oops, I didn't noticed, sorry. I'll check twatch.py next time I touch python 
binding codes.

BTW, how about simply adding its own eprintf implementation?


> Other than that, yeah, I think perf_attr_conf is needed and the rest of
> the patch looks ok, will look again after you address the above
> comments,
>
> Thanks!
>
> - Arnaldo
>

Thanks for your review,
Namhyung


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