Re: [PATCH 1/6] perf/core: Provide a kernel-internal interface to get to performance counters

Frederic Weisbecker
2009-11-02T13:00:16Z

On Mon, Nov 02, 2009 at 09:47:49PM +1100, Paul Mackerras wrote:
> Arjan van de Ven writes:
>
> > On Mon, 2 Nov 2009 14:46:55 +1100
> > Paul Mackerras <paulus@samba.org> wrote:
> >
> > > Frederic Weisbecker writes:
> > >
> > > > /*
> > > > + * perf_event_create_kernel_counter
> > > > + * MUST be called from a kernel thread.
> > >
> > > Why? The reason for this requirement isn't obvious to me. It would
> > > be good to have the reason documented in the comment for the sake of
> > > people modifying the code in future.
> >
> > because if you call it from another context it will attach to that
> > context... and go away when that context goes away...
>
> I don't think that's right. When a task exits, that doesn't
> automatically kill all the perf_events it created. The perf_events
> each have a reference to their owner's task_struct, so the task_struct
> will hang around until all of the perf_events get released. Normally
> top-level perf_events have an associated filp and we use its reference
> count to control the perf_event lifetime, but with these new kernel
> perf_events there is no filp, so the caller will have to do any
> refcounting required.
>
> Bottom line is that a perf_event created by
> perf_event_create_kernel_counter will exist until someone calls
> perf_event_release_kernel on it, whether or not the owner task exits
> (and whether or not that task is a kernel thread or a usermode
> process). So I see no need to prohibit creating kernel perf_events in
> the context of a user task, though callers need to be aware that doing
> so could potentially mean the user task's task_struct has to hang
> around for a long time after the task exits.
>
> Paul.

Yeah. For example while creating a breakpoint through ptrace,
the owner of an event created by perf_event_create_kernel_counter
is the user task that does the ptrace call.

But we need to explicitly release some disabled/pending ptrace
breakpoints using perf_event_release_kernel when the task exits.
I should simplify this part in the future.

That said, it's all fine to set the owner as a user task like in this
example.

Should I remove this comment?