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

[Yann Droneaud <ydroneaud@opteya.com>]

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