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

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Date:	Mon, 2 Nov 2009 21:47:49 +1100
From:	Paul Mackerras <paulus@...ba.org>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	"K.Prasad" <prasad@...ux.vnet.ibm.com>,
	Alan Stern <stern@...land.harvard.edu>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Jan Kiszka <jan.kiszka@...mens.com>,
	Jiri Slaby <jirislaby@...il.com>,
	Li Zefan <lizf@...fujitsu.com>, Avi Kivity <avi@...hat.com>,
	Mike Galbraith <efault@....de>,
	Masami Hiramatsu <mhiramat@...hat.com>,
	Paul Mundt <lethal@...ux-sh.org>,
	Jan Kiszka <jan.kiszka@....de>
Subject: Re: [PATCH 1/6] perf/core: Provide a kernel-internal interface to
 get to performance counters

Arjan van de Ven writes:

> On Mon, 2 Nov 2009 14:46:55 +1100
> Paul Mackerras <paulus@...ba.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.
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