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.zijls...@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?
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.
Vince
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