linux-kernel - Re: [PATCH] perf tools: enable close-on-exec flag on perf file descriptor

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Date:	Mon, 6 Jan 2014 18:14:44 -0300
From:	Arnaldo Carvalho de Melo <acme@...stprotocols.net>
To:	Yann Droneaud <ydroneaud@...eya.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Paul Mackerras <paulus@...ba.org>,
	Ingo Molnar <mingo@...hat.com>, Jiri Olsa <jolsa@...hat.com>,
	Namhyung Kim <namhyung@...nel.org>,
	Andi Kleen <ak@...ux.intel.com>,
	David Ahern <dsahern@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Mike Galbraith <efault@....de>,
	Stephane Eranian <eranian@...gle.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Michael Ellerman <michael@...erman.id.au>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] perf tools: enable close-on-exec flag on perf file
 descriptor

Em Mon, Jan 06, 2014 at 10:01:35PM +0100, Yann Droneaud escreveu:
> Hi,
> 
> Le lundi 06 janvier 2014 à 11:43 -0300, Arnaldo Carvalho de Melo a
> écrit :
> > Em Mon, Jan 06, 2014 at 12:24:36PM +0100, Peter Zijlstra escreveu:
> 
> > > acme, ACK? I was thinking I'd keep these two patches together so the
> > > entire things lands in tip in one go?
> > 
> > Nope, it should notice the EINVAL, drop the flag that doesn't work on
> > older kernels, retry, so that new tools continue to work on older
> > kernels, with yet another fallback.
> > 
> 
> But it may be difficult to distinguish the 'origin' of EINVAL: is it
> from FD_CLOEXEC flag or from an unsupported parameter in the attributes.

It definitely is, that is why the first thing to fall back is the most
recently added feature.
 
> > Please take a look at __perf_evsel__open(), probably it will be best to
> > add a flag to perf_missing_features, like the one we have for the
> > perf_event_attr.mmap2 flag, so that we fail just once, etc.
> > 
> 
> Unfortunately perf_event_open() is called in multiple places, not only
> in __perf_evsel__open(). So a more generic solution should be designed.
> 
> Is something like the function proposed in message
> <1389022310.13828.9.camel@...alhost.localdomain> [1]
> ok to be added to its own module:

Well, at least make it set a perf_missing_features flag instead of that
'cloexec' global, that way the routines that use perf_evsel__open would
reuse it.

- Arnaldo
 
> static int cloexec = PERF_FLAG_FD_CLOEXEC;
> 
> int perf_flag_fd_cloexec(void)
> {
>     static int probed;
> 
>     if (!probed) {
>         struct perf_event_attr attr = { 0 };
>         int fd = perf_event_open(&attr, 0, -1, -1,
> PERF_FLAG_FD_CLOEXEC);
>         probed = 1;
>         if (fd >= 0)
>             close(fd);
>         else
>             cloexec = 0;
>      }
> 
>      return cloexec;
> }
> 
> This function should be used to build the flag passed to
> perf_event_open().
> 
> Regards.
> 
> [1] 
> http://lkml.kernel.org/r/1389022310.13828.9.camel@localhost.localdomain
> 
> -- 
> Yann Droneaud
> OPTEYA
> 
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