Messages in this thread | | | Date | Fri, 9 Jan 2015 17:00:51 +0100 | From | Peter Zijlstra <> | Subject | Re: perf: PERF_FLAG_FD_OUTPUT has been broken since 2.6.35 |
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On Thu, Jan 08, 2015 at 10:22:37AM -0500, Vince Weaver wrote: > Hello, > > I was working on improving the manpage by working out how the > perf_event_open() PERF_FLAG_FD_OUTPUT flag works. > > It turns out it doesn't. > > In kernel/events/core.c when opening a file this is done: > > if (output_event) { > err = perf_event_set_output(event, output_event); > if (err) > goto err_context; > } > > and perf_event_set_output() does: > > if (output_event->cpu == -1 && output_event->ctx != event->ctx) { > goto out; > } > > But event->ctx is always NULL; it does not get set to ctx until > after the call to perf_event_set_output(). > > It looks like this was broken back in 2.6.35 days with the change: > > commit ac9721f3f54b27a16c7e1afb2481e7ee95a70318 > Author: Peter Zijlstra <a.p.zijlstra@chello.nl> > Date: Thu May 27 12:54:41 2010 +0200 > > perf_events: Fix races and clean up perf_event and perf_mmap_data interaction > > So is this worth fixing seeing as apparently no one uses this feature?
I think there's a fair argument for removing it, Ingo, Acme?
> As an aside, added error reporting is *really* needed as I had to sprinkle > all of events/core.c with printk() calls to even begin to understand why > I was getting the EINVAL result.
Yes, let me see if I can find someone to work on that :/
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