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

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Date:	Mon, 06 Jan 2014 16:31:50 +0100
From:	Yann Droneaud <ydroneaud@...eya.com>
To:	Jiri Olsa <jolsa@...hat.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Mackerras <paulus@...ba.org>,
	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	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,
	Peter Zijlstra <peterz@...radead.org>,
	Yann Droneaud <ydroneaud@...eya.com>
Subject: Re: [PATCH] perf tools: enable close-on-exec flag on perf file
 descriptor

Hi,

Le lundi 06 janvier 2014 à 15:22 +0100, Jiri Olsa a écrit :
> On Mon, Jan 06, 2014 at 11:51:25AM +0100, Yann Droneaud wrote:
> > In a previous patch [1][2], flag PERF_FLAG_FD_CLOEXEC was
> > added to perf_event_open(2) syscall to allows userspace
> > to enable close-on-exec behavor atomically when creating
> > the file descriptor.
> > 
> > This patch makes perf tools use the new flag.
> > 
> > Beware that perf tools compiled with the new flag won't work
> > on older kernel which do not support flag PERF_FLAG_FD_CLOEXEC.
> 
> I think we should enhance the api probe routines (perf_do_probe_api)
> to detect that, than just bypass us from running on older kernels
> 

perf_probe_api() is only available in tools/perf/util/record.c module
(eg. it's a static function), while I've modified all calls to
perf_event_open().

So if you want to follow the probe path, a new function should be added.
This function must be used to retrieve the value of the
PERF_FLAG_FD_CLOEXEC flag if available.
For example:

static int cloexec = PERF_FLAG_FD_CLOEXEC;

int perf_flag_fd_cloexec(void)
{
    static int probed;

    if (!probed) {
        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;
}

But I don't know how to setup a struct perf_event_attr which will work
in "all" case (and do no harm).

I was able to run this code with attr struct cleared (eg. set to 0), but
I don't know perf internals enough to be confident.

Regards.

-- 
Yann Droneaud
OPTEYA


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